Length is probably a factor. It packs a lot of nice prose in a slim little volume. Its simple story and characters are, at least, more complex than your average

Length is probably a factor. It packs a lot of nice prose in a slim little volume. Its simple story and characters are, at least, more complex than your average pulp adventure tale—combine that and the length and it's just about an ideal Baby's First Real Literary Novel, which is probably why it's assigned so much in high school and 100-level undergrad literature courses. I also take a bit more depth from between the lines in the novel, than it may always get credit for, but that may just be me. Largely due to Fitzgerald sticking (perhaps too much) to his perspective character's limitations, there's a lot happening just off the page. A lot of suggestion, a lot left to the reader to fill in, which I personally enjoy.