Most Helpful Girl
There could be multiple reasons for this:
She REALLY just doesn't want to be more than friends.
However, since she has let kissing happen, it is possible that she is very comfortable (intimately speaking) with your personality, but doesn't want to date because she is not Physically attracted to you. (However, I cannot judge you attractiveness and if you guys messed around that is unlikely.)
Or the exact opposite, she is really attracted to you, but personality wise she doesn't see you as relationship material? And she doesn't want to lead you on. (My friend has this problem)
Or maybe she is just embarrassed and doesn't want you to look at her as someone who is 'easy'.
Even I don't understand all the complications that are girls, and I am one.