Kids are being kept in diapers longer now than before.
In the old days before disposable diapers kids were diapered with cloth diapers and plastic pants which were uncomfortable for the kid to wear and when they wet the cloth diaper they would feel the wetness.
So the kid would have an incentive to stop wearing those wet cloth diapers as they didn't feel good.
When the kid peed in the cloth diaper it would get the skin wet and the kid would basically be sitting in their pee.
But now with the invention of disposable diapers and the fact that the disposable diapers are getting better at absorbing the pee and keeping the skin dry the kid does not feel uncomfortable in the wet disposable diaper.
So the kid does not have any reason to want to get out of diapers.
Peeing in and wearing a wet disposable diaper is very different than peeing in and wearing wet cloth diapers and wet pants.
The gel in the disposable diapers absorb the pee and keep it away from the skin.
While the kid may feel slight wetness against their skin when the diaper gets full they do not feel all that wet.
Kids have grown to find diapers comfortable now which makes it harder to potty train.
It used to be that kids were beginning potty training by age 18 months to 2 years but now some kids are still wearing diapers full time until 3 and even 4 years of age.
Even some kids are still wearing diapers full time and not potty trained by age 5 to 6 years old.
Eventually the kid will potty train but disposable diapers make it harder.