Nothing (makes) a man commit to a woman. Commitment like love and affection are things that are given (freely). Promise + Time = Commitment
Everyone seems to have their own definition of commitment. Some people feel anything less than marriage is an "un-commited relationship" and there others who have lived together for 20 years in a happy monogamus relationship that consider themselves to be in a "commited relationship". What may cause one man to decide he wants to get married will vary from man to man.
Keep in mind marriage/weddings are not something most men grew up dreaming about as little boys. In fact at most weddings the groom is little more than a prop. The audience only gets on it's feet when the bride walks into the room. A man who is "in love" with a woman is not going to (love her more) because her title has been changed from girlfriend to wife. Marriage is not a priority for most men and especially those under the age of 32.
In fact couples who are dating in an "exlusive relationship" have the same expectations of one another as those who are married. With a divorce rate of 50% one can hardly say being married is more commited than living together. Awhile back I wrote a hub on this topic. Commitment Without Marriage: Is it possible? https://hubpages.com/relationships/CommitmentWitho...