There isn't much point in marrying someone when you're already getting everything anyway.
If you are already living with a man, sharing bills, have children together, there's no point getting married, not for the man, he already has everything that marriage brings.
The question isn't "why won't he marry me".
You should be asking "why should he marry me".
What does marriage get him that he doesn't currently have?
Might sound a bit transactional, but it's accurate.
I also think that waiting around for a man to propose is not a great situation to stay in. If he's the marriage type, if it's important to him, (and he decides you're the one for him) he won't wait years to make it happen.
Basically, if he hasn't asked you, he doesn't want you, no matter how many kids, bills or cars you might share.