Because he has already told you he isn't, he likes someone else. While he is interested in her he is passing time with you, and you appear to him to be okay with that.
You asked about a relationship and he made his feelings clear, we know this because you told us.
You are better than that. I say this because it's clear that you want more but are playing the casual to be sure he comes to you.
You're lying to him but more importantly, you're lying to yourself.
Let it go, you'll feel like crap for about 6 months then I promise you, it'll be %better.
You will have more happy years without him than this that you're having with him