- Being part of a community by reading something lots of people are reading

- Being a new reader that hasn't really developed a sense of taste yet, so you read what's popular

- These books are literally marketed to certain groups of people (themes, cover design, ...) and people are easy victims

- They want books they can identify with

- They want to read something they don't really have to think about in a deep, philosophical way

- If you don't have lots of time to try out new books, it's easy to pick something that "most people like", so as not to waste time on a "bad book"

That's probably it.