Youngman Carter painted beautiful bespoke art for the covers of Margery Allingham's mystery novels, and his style became associated with her work. Why don't the modern reprints of the Campion novels use her husband's art?
There could be numerous reasons:
If published under a different imprint, the new publisher may not have the rights to the original artwork.
Response to artwork is subjective, so a new editor may have may see value in a different cover, based on the current market. (People like new, shiny stuff!)
It may simply be a way to distinguish the printing from earlier printings.
It would be helpful if you could post or link to pictures of the same novel with different covers. My own personal pet peeve is when a movie based on a book comes out, and instead of the wonderful, original cover, the work is branded for years with a horrible, movie-poster cover.