Something About The Parents

There is something going on in popular culture right now that I'm trying to figure out. And it's this: an astonishing number of crime-related shows now feature the older single parent of the protagonist.

Now, I have nothing against this: for one thing, it's giving a lot of work to people who were on TV when I was a kid, and this provides a sort of reassuring continuity every time I turn on the TV. 

And I suppose there might be nothing more than imitation going on here: somebody saw this in a show they liked and decided to put it in their show too.  (There also seems to be some sexism going on: the single dads mostly get to dispense words of crochety wisdom, while the single moms get to be whiny pains in the ass.)

Crossing Jordan, featuring Ken "The White Shadow" Howard as whatsherface's dad with a laughably bad Boston accent was the first one I remember seeing, but since then, we've gotten:

Burn Notice- Sharon "Cagney & Lacey" Gless as Michael's mom!

Psych-- Corbin "L.A. Law" Bernsen as Sean's dad!

Prime Suspect-- The fat guy from Homicide as, um, the main lady's dad!

Rizzoli & Isles-- Lorraine Bracco from the Sopranos as Jane's mom!

Castle--Susan Sullivan from It's a Living and Falcon Crest as Castle's mom!

Here are my questions: why are the older parents always single? (Notable exception: The Closer, but her parents haven't been regular characters)

Why are so many writers using this idea, and why always in the crime shows? 

Is there no room for Carl Lumbly in this trend? Or Daniel J. Travanti? Or Pam Dawber?

What does it all mean?