This is the kind of information that would have been helpful to know in your original post
From what you said here, I'd guess that either this guy is married or has someone else (as others have stated), or that the being friends first and taking things slowly wasn't his cup of tea.
While there are certainly plenty of men out there who see the sense of moving slowly in a relationship, there are also a lot of guys who will agree to move slowly but don't really want to. As in, maybe they think they can talk the woman into moving faster, or they don't think the woman is serious about the pace she claims to want to go.
It's possible he just got bored- which isn't your fault.
In any case, any way you look at it, it sounds like he did you a favor by taking himself out of the picture. Now you're free to move on; here's hoping you find what you're looking for