I personally wouldn't date a guy I wasn't into, but some females would and do and guys do it too. I only ever would accept an invite and if it was an invite from a guy I was really into and because if it's a guy I'm not into, it sends mixed messages and to date him.
I could be wrong, but isn't that what 'dating' is all about in the States? You date random people and to see if anything clicks and it's not necessarily a guy/girl that you are into that you date?
Maybe she did have a good time, but something just didn't 'click' with her. This happens all the time. I've read a zillion stories like yours in forums, where the guy and girl have dated, everything seemed to be going great, etc, etc....and then the other person didn't call, or said they'd like to leave it as friends.
It's just one of those things that we experience and we move on from it.
Doesn't mean there is anything wrong with you, just that she didn't think you were right for her and for whatever reason and there could be a thousand reasons. Maybe her ex, or some other guy even came back in the picture.