What goes in the mind of 40+ and unmarried? Who knows. Will ask and answer specifically! 
However, not being married for a while may be due to many reasons including
Being too rigid in his ways.
Not being able to find a good match
Being too involved with day to day life
Being psychologically afraid of being commited to one person or has some psych disease- some people love the single life being alone at home and talk to the walls…!!
Some prior mishaps
Remembering someone from past
He is seeing someone secretively and does not feel the need to get married
If not gay then bisexual…you said you know! I am iching to know how do you know. j/k.
He may surprise you and get married in next few months