TL;DR: The Longbourn estate is ‘entailed’ to male heirs only, whereas Rosings is not.
Austen sets out the financial situation of the Bennets in detail:
Mr. Bennet’s property consisted almost entirely in an estate of two thousand a year, which, unfortunately for his daughters, was entailed, in default of heirs male, on a distant relation; and their mother’s fortune, though ample for her situation in life, could but ill supply the deficiency of his. Her father had been an attorney in Meryton, and had left her four thousand pounds. [Chapter 7]
Mr. Bennet’s main asset was thus an estate of land at Longbourn, which generated an income of £2,000 a year by renting it out to tenant farmers. The estate was ‘entailed’, meaning that in law Mr. Bennet was a ‘tenant in tail’: he could make use of the estate while he was alive, but he was not allowed not sell the land, and he could not dispose of the estate in his will. Instead the estate would pass at his death to the next male heir in line of the landowner who originally created the entail. Since Mr. Bennet had no male heirs, the estate would pass to his cousin Mr. Collins (except in remote circumstances, such as Mrs. Bennet dying, Mr. Bennet marrying again and having a son).
The entail only covers the Longbourn estate, and does not include Mr. Bennet’s other property, and so if he had been more prudent, he could have saved money out of his income from the estate in order to support his family after his death:
Mr. Bennet had very often wished before this period of his life that, instead of spending his whole income, he had laid by an annual sum for the better provision of his children, and of his wife, if she survived him. [Chapter 50]
However, he did not do this, because his plan for providing for his family was to ‘cut off’ (or ‘bar’) the entail:
When first Mr. Bennet had married, economy was held to be perfectly useless, for, of course, they were to have a son. The son was to join in cutting off the entail, as soon as he should be of age, and the widow and younger children would by that means be provided for.
Lacking a son, he was apparently unable to carry out this plan:
Five daughters successively entered the world, but yet the son was to come; and Mrs. Bennet, for many years after Lydia’s birth, had been certain that he would. This event had at last been despaired of, but it was then too late to be saving.
Austen does not give explicit details of the legal and financial situation of the de Bourgh family, but there are some clues. First, we learn that Lady Catherine de Bourgh is a widow and her daughter Anne is the “heiress of Rosings”:
[Mrs. Bennet] “I think you said she was a widow, sir? Has she any family?”
[Mr. Collins] “She has only one daughter, the heiress of Rosings, and of very extensive property.”
“Ah!” said Mrs. Bennet, shaking her head, “then she is better off than many girls.” [Chapter 14]
It seems that Rosings is not entailed:
[Lady Catherine] “Your father’s estate is entailed on Mr. Collins, I think. For your sake,” turning to Charlotte, “I am glad of it; but otherwise I see no occasion for entailing estates from the female line. It was not thought necessary in Sir Lewis de Bourgh’s family.” [Chapter 29]
We can therefore guess that Sir Lewis de Bourgh disposed of Rosings in his will, either by leaving the whole estate to his widow, or else by giving his widow a life interest, with the estate reverting to their daughter on Lady Catherine’s death.
Entails were widely considered unjust (especially by owners of entailed estates who wished to raise money by selling off parts of the land), and were effectively abolished by §176 of the Law of Property Act 1925.
How realistic is Austen’s portrayal of the legal situation?
Black’s Law Dictionary explains how the entail could have been barred:
barring of entail. The freeing of an estate from the limitations imposed by an entail and permitting its free disposition. This was anciently [that is, prior to the Fines and Recoveries Act 1833] done by means of a fine or common recovery.
So why did Mr. Bennet believe that, lacking a son, he was unable to bar the entail, given that the procedure of common recovery was available to him? Some legal scholars think that Austen’s account of the legal situation of the Bennets was not realistic on this point. According to Peter Appel:
At the time that Austen wrote, it would have been extremely unlikely that a landed family like the Bennets would have used the entailment standing alone as the legal means of keeping Longbourn within the family. More likely, they would have used a device known as the strict settlement. It was also extremely rare (although not impossible) that a strict settlement would have been arranged to cut off close relations like the Bennet daughters. If the restriction on Longbourn was an entailment standing alone—which would have in all likelihood cut off any provision for the Bennet daughters—then the current life tenant (i.e., Mr. Bennet) could have ‘barred the entail’. This term means that Mr. Bennet could have stopped the property from going to Mr. Collins through a fairly simple legal proceeding. After that, he could have left it to whomever he wished: Jane, the eldest daughter; Elizabeth, his clear favorite; or all five of his daughters in whatever shares he chose.
Peter A. Appel (2013), ‘A Funhouse Mirror of Law: The Entailment in Jane Austen’s Pride and Prejudice’, Georgia Journal of International and Comparative Law 41, p. 609.
What are we to make of this discrepancy? There are various possibilities:
Austen was using dramatic licence to exaggerate the legal predicament of the Bennet family.
The characters’ descriptions of the entail were somewhat loose or inaccurate (as might be expected, since they are not lawyers) and in fact Mr. Bennet was not the tenant in tail of Longbourn, but only had a life interest in the estate, and so common recovery was not available to him. This situation could have arisen if Mr. Bennet’s grandfather had owned the estate in fee simple, and if Mr. Bennet had been living at his grandfather’s death. Then his grandfather, in his will, could have given life interests to both Mr. Bennet’s father and Mr. Bennet, with the remainder left in fee tail male. (This would be a form of ‘strict settlement’ as discussed above.)
Austen was using the discrepancy to comment on the character of Mr. Bennet:
Nevertheless, the more intriguing scenario for contemplating the relationship between law and society is if both Austen and her readers knew that an entail in England could be barred. If this were the case, two interrelated implications immediately leap forth. First, the character of Mr. Bennet must be reread. This point is probably of more interest to Austen scholars and fans. Most readers generally sympathize with Mr. Bennet because he is largely surrounded by folly, because he is witty, and because he favors Elizabeth, the heroine, and recognizes her intelligence. These features—especially that he can find intelligence in a woman, and particularly in the early nineteenth century—are attractive to a modem audience. They make him seem urbane and progressive. A Mr. Bennet who could, but did not, provide for his daughters, however, becomes a much less appealing character. Why would Mr. Bennet not disentail the property if he could? Perhaps it was simply not done, or not an option to a family of the social class or status ofthe Bennets.
Answers to questions raised in comments
Why were entails created? The purpose was to maintain the concentration of power and wealth (as embodied in an estate of land) by preventing it from being sold off or divided up among many heirs.
Why were entails so often restricted to male heirs? No doubt there was a substantial degree of sexism involved, but additionally, prior to the Married Women’s Property Act 1870, a woman in England lost control over her property after marriage: she could not sell, lease, or mortgage her estate without her husband’s consent. For the landowner creating the entail, this risk of loss of control could be avoided by specifying that only male heirs could inherit.
What about the family name? A landowner who was concerned about the preservation of his surname could require, when creating an entail, that a male heir succeeding via a female line must adopt his surname. For example, Jane Austen’s own brother Edward changed his surname to Knight as a condition of inheriting the estate of his relative Catherine Knight. Perhaps the ancestor of Mr. Bennet who created the Longbourn entail was not so concerned; alternatively, Mr. Collins may be required to change his name, but this is not mentioned in the novel.