At the time, there were many Jewish bankers/moneylenders, so moneylenders were probably often portrayed as Jews.
For a very long time, the Jews have been associated with banking and money. There are a few reasons, such as that there were a lot of Jewish bankers after the 11th century. In The Encyclopedia Judaica, in their article on Banking and Bankers, in the section on Europe, they say:
After the First Crusade, (1096) the Jewish merchant, in his necessarily long journeys, no longer enjoyed even minimal physical security. In Western and Central Europe, especially in Spain, the crystallization of the essentially Christian nature of the rising city communes combined with this insecurity to drive out the Jews from commerce and prohibit them from engaging in crafts. In France, England (up to 1290), Germany, Austria, Bohemia, Moravia, and northern and central Italy, Jews had to turn to loan-baking on a larger or smaller scale to make a living. Emphasis mine.
So already in the 11th century, the Jews were turning to money lending. It continued...
By the 13th century the notion that the Wucherer ("usurer") was a Jew was already current, for example, in the writings of Berthold of Regensburg, Walther von dee Vogelweide, and Ulrich von Lichtenstein. The word judaizare became identical with "taking interest".
Moneychanging and coinage privileges were often combined with moneylending, and Jews were frequently the sole agents arranging loans...
The article goes on to describe more about the centuries afterwards, but let it suffice to say that there were many Jewish bankers. Let's skip to the section on 19th and 20th centuries.
The article describes many well-known Jewish bankers, such as the Rothschilds, the Mendelssohns, and the Bleichröeders. These families all made large loans at times, and were fairly famous.
All of those families have Wikipedia articles, as well as articles in the Encyclopedia. I'll quote from the beginning of the Wikipedia articles:
During the 19th century, the Rothschild family possessed the largest private fortune in the world, as well as the largest private fortune in modern world history. The family's wealth was divided among various descendants and today their interests cover a diverse range of fields, including financial services, real estate, mining, energy, mixed farming, winemaking and nonprofits.
In 1795 the eldest son Joseph Mendelssohn established the bank Mendelssohn & Co. in Berlin, and his brother Abraham joined the company in 1804. Many members of the family worked for the bank until it was forced to shut down in 1938.
Bleichröder was born in Berlin. He was the eldest son of Samuel Bleichröder, who founded the banking firm of S. Bleichröder in 1803 in Berlin. Gerson first joined the family business in 1839. In 1855 upon the death of his father, Gerson became the head of the banking firm. The bank maintained close contacts with the Rothschild family; the banking house of Bleichröder acted as a branch office in Berlin of the Rothschilds' bank.
In an article in Medieval Jewish Civilization: An Encyclopedia, reprinted online at myjewishlearning.com, they say:
Ultimately, however, the potential of great profits and the widespread demand for moneylending made it universal among Jews. Mordecai B. Hillel of Germany (b. 1298) wrote that there is no profit in any form of commerce like that to be made in lending money. Ibn Adret in Spain observed that it has become permitted for everyone to charge interest on loans to Gentiles, “and now all have made themselves ‘sages’ in this respect, adding that he heard in the name of Rashi, that this is because taxes have constantly been increased and there is no longer any limit to “because of livelihood” (i.e. in order to meet their tax burden, Jews had no alternative.)
As we see above, after they started the moneylending, they continued.
We cannot ignore the influence of The Merchant of Venice, by william-shakespeare. In it, a Jew named Shylock is a moneylender who wants to 'take a pound of flesh' as payment. (There were actually no Jews in England at the time of the writing of this play, as they were expelled in 1290 and didn't return until 1657, and The Merchant of Venice was apparently written between 1596 and 1599.) As the play has been regarded as the play in which Shakespeare 'found himself the most' (source: Encyclopedia Judaica's article on Shakespeare), it was fairly well read. So a lot of people had this idea that Jews were moneylenders while there weren't even any Jews there, and people held that in mind for a while, and then it gained more credibility when the Rothschilds and the other bankers came on the scene.