I have seen her posted a lot, and kind of wondered the same thing. So here is my take on it:

-She played Hermione who was a great female character. And that book series was wildly popular. She is still associated with the quirky, bookish nature of Hermione.

-Many of us kind of grew up with her. It started with Hermione in the books, then she became the face and each year we watched her grow from a kid (like us), to a bit awkward of a teen (like us), and into an attractive woman.

-There is certainly some of the "circlejerk" or herd aspect to it. She has a big following which gets her more exposure, and that leads to more exposure and more and more. She is like a guaranteed upvote on Reddit.

-Back to her acting and character, she ended up with Ron. He is a normal, less than super-hot guy. Yes, it's just a book/film but this immediately made her accessible to a huge range of less than super-hot guys.

-Her image is that of an intelligent and strong woman which is certainly very attractive.