Most Helpful Guy
There are heaps of possible explanations, work commutes, friends, is he not that serious with you? are you difficult to live with? Have you discussed marriage/ engagement? Is he threatened by moving into 'your' place?
If you're serious about him, I suggest you have the talk and see where he's at..
Most Helpful Girl
Have you talked about marriage at all? This may/not be the reason. Some guys see moving in as a "first step" to getting married. And he may not want to do that yet.
Or he could simply love his privacy. I know I do.