I guess I'm old fashioned but I will give you my take.
Why SHOULD he marry you? He has you. You live together. You are together all the time. What would be different if you were married? Nothing? Well, except for that little thing called COMMITMENT.
I'm not trying to be harsh, but he has a great deal. He has you living in with no legal commitment at all! I don't buy into the theory that it's just a "piece of paper." It's much more than that. It's protection for you legally. Otherwise you are just roommates.
I know it's a cliche - but why should he buy the cow when he gets the milk for free?

It's the same concept.
I know that 'everyone' lives together now. And sometimes it works out. Sometimes it doesn't. Just like marriages. Half of them don't work. But there is something to be said for doing things in order and WAITING to move in together until you are legally and totally committed. And that happens with a wedding.
My guess is that he has no interest in marriage now. After all, look at it from his point of view. What's the point really?
Just my opinion of course.