Well likely a number of them are simply divorced, and divorced and in your 40s is a bit of a cliché for a reason. This puts many of them in recovery mode.
We also have anti-male/female groups openly discouraging marriage, even among Christians.
Also teachings among Christians on finding a mate has raised people's standards to near perfection.
As well as a rise in speaking against dating at all, instead teaching things like courtship. This greatly limits your available options as it is the unpopular view.
Unrealistic views on dating and marriage. The number of 18-25 year olds depressed and hopeless because they aren't married or dating is surprising. Such people are likely to remain single because their desperation is obvious, or end up in bad marriages, in which they'll be divorced and reluctant to remarry.
Not to mention the frequency of threads started here on how someone can't find a 'real' Christian to date/marry. Ironically the male to female ratio to these complaints are fairly even.
I'm sure there is more to go on but all these things make for a lot of explanation about being single in your 40s.