Because Country is more internally-warm than pop or rock. Country music is a story. Country artist are story tellers and you feel better listening to country. For a girl who was lonely in school, country is just what she needed.
Now, for any other reasons I could guess to, I don't know. Besides, how the heck are we supposed to know how Taylor feels? She's a country artist. She's a born story teller. And that's it.
She grew up with country music, and it's just her style. If you don't like it... Well, that's kinda your problem. :p Taylor Swift likes country music, with a little bit of pop. That's just what she likes. Nobody can really change that.