Family psychologist, Dr. John Rosemond has weighed in on this one and he (I think this is the right word) blames it on ultra-absorbent diapers because it steals the cue to learn (loss of the sensation of disagreeable wetness). I think he cites some research on the problem (that, in fact, the age of continence is rising).
I think this is a classic example of the way family needs and desires evolve over time. Because mother's do not wash poopy and wet diapers themselves and because disposables work so well, there is less impetus to get the kid out of diapers. Now, I have seen families notice that buying disposables for 2 kids was way too expensive and suddenly it was TIME to potty train the elder. Bet you've seen that too, Enright.
All I know is that, even though I periodically offered it, my son didn't care about being wet or poopy until he was three and then he potty trained. And I wasn't willing to make it a hill to die on before that time.
My opinion is that there has to be some physiologic things in place before a child can attain bladder control. One pediatrician I know (an old guy) said that in his experience a child didn't attain that until he could pedal a tricycle, so he used that as his signal to say, "Time to potty train." Some kids HATE the sensation of being wet or poopy and they figure it out really quick. My kid pooh-ed in his pants (not diapers) ONCE and I gave him the opportunity to clean out his own underwear in the toilet. Gee, it never happened again. So I think that says something about ownership of the problem (which is a big thing for Rosemond, too). Most resources say the kid must have a word for urination or pooping to be ready, too.
I think the fact that families with working moms and dads spend so little "ordinary" time together is a contributor, too. Potty training requires being able to connect the stimulus of the full bladder with going to the bathroom (in a timely fashion) and most kids are going to be most comfortable with their little kid potties. So many families are constantly on the go and it is simply more expedient to let the kid pee or poop in the diaper. This mixed message of pee in your diaper at the grocery store or the soccer game, but tell me at home so we can get you to the toilet is too confusing.
If you want to read more about Rosemond on potty training, he does have a web site and might have some of his opinions there, though I cannot say for sure. Again, to me this has as much to do with the kid as the family and when they are willing to "work on it."