I think it depends on the individual and their situation. Personally my first thoughts would be that they have some emotional issues/commitment issues/self-esteem issues, although that's not necessarily true.I have two friends, one late 30s, the other early 40s. One has never had a relationship at all, serious or otherwise, and the other has only had one semi serious relationship.
The guy who has never had a relationship has some serious self-esteem issues and confidance issues, and he has a fear of being in a relationship because of the pressures and strife that he's seen occur in other people's relationships - he views it as potentially more trouble than it's worth. He's a Pisces/Gemini Moon/Virgo Rising.
The other friend has had a couple short-term, semi serious relationships. He doesn't have commitment issues, he's just your super quiet techie type who isn't often in a position to meet women, and he's become very self contained and self sufficient so he doesn't seem to be actively looking. To be honest, I've never really been able to figure out what his deal is. He's Scorp/Aries Moon/Sag Rising.
My step father also didn't really ever have a serious relationship before he met my mom when he was in his mid-late 40s. What is interesting in his situation is that he's pretty outgoing and chatty - usually not typical for the other guys I know who were single that long. A good deal of his younger years were spent focused on his business and traveling, and he never wanted kids really. He's Cappy (I don't know what his other aspects are).
I would blame the phenomenon more on self esteem issues and lack of confidance than than on an unwillingess to commit. I've known commitment-phobes, those guys usually were "playas", who just couldn't/wouldn't commit to only one woman.
edit: Interestingly, I've never met a woman who has never had any kind of serious/semi serious relationship. I've known some Virgo women who had a serious relationship or two and afterwards just kinda quit dating altogether for decades, but never even heard of a woman in her 40's who has never had a serious relationship (except maybe nuns?). /shrug