-Very skilled songwriter. I don't necessarily like all her stuff but I legitimately think she's the best songwriter (meaning, composing music and writing lyrics) of her generation. Probably since Bruce Springsteen or Billy Joel.
-Very attractive and a good performer
-Well connected in the music industry let her get an early start/inside track
-obsessed with being popular. I don't mean that in a negative way, but her primary objective with her music is to please as many people as possible. I think the documentary "Miss Americana" on Netflix explains that very well-at one point she straight up says "I just want people to like me" or something like that. That means her music/career has always focused on mass appeal as opposed to making more... limited-appeal music like most artists do at some point in their career
-she's kept a remarkably clean image even through being famous for close to two decades. It's very telling that the worst thing her haters can say about her is "but her plane uses a lot of carbon!" This means parents let kids listen to her, brands love her as a sponsor, nobody boycotts her, etc.
-one last thing, I think people love her songs because they feel like they're true. Her songs have a very intimate, almost confessional quality that a lot of artists strive for buy often comes off as fake.