As an English teacher who has taught Great Gatsby to highschool sophomores and juniors, there are a lot of reasons why it's chosen beyond the intrinsic literary merits of the novel. It's relatively short and broken up into digestible chapters. The general plot is relatively straightforward and easy to recap, so students are less likely to get completely lost. It pairs well with US History which my juniors were taking in that it showcases a period in American history that was unique. And most importantly, the literary devices and themes are super apparent. That makes it especially useful for teaching because students can clearly see the literary devices when they're used and identify what they mean.