It's a mixture of things, really. 1.The uandishi style for one - it is very bland and when kusoma I get the impression Meyer has clicked 'synonyms' on Word and switched words for longer ones that don't really fit. 2.The characters, too. I find Bella whiny and quite manipulative, but everyone seems to find her perfect, so I'm constantly thinking 'Mary Sue' when reading. I find Edward to be controlling and arrogant and extremely patronising, and I dislike his actions in regard to these character traits are seen as romantic. 3.The number of times Edward is compared to Adonis. In fact, the ridiculously high number of times one of the Cullens is described as being beautiful. 4. Renesmee. 5. Imprinting. 6. The lack of any tension in the plot, the lack of any character death, the lack of anything remotely relatable. 7. The way Bella treats her friends.
Yes, the romance is definitely the main reason, but I think there's zaidi to it than that. Well, first of all, guys have a reputation to keep in check. In all seriousness, being caught kusoma Twilight is a great way for guys to lose their pride. And also, Twilight is just one of those things that's so easy to make fun of...there's so many "dumb" things in the movies/books, and guys always jump at the chance to laugh at something.