The thing is, potty training a child can be hard work. Many children would far prefer to be in nappies, and not interrupt their play etc, than have to leave what they were doing to use a potty, so there is very little 'in it for them'.
In the past, there were counterbalances for that hard work, and more in it both for the child and for the parent - less washing, ability to access pre-schools (Ds and DD, now teens, had to be potty trained to attend pre-school from 2.5), so more people put that hard work in at earlier points.
Now, the points where there are counterbalances tend to kick in later. The 'well, i have to bite the bullet now' age has moved to school age, rather than pre-school age. Yes, some children are easier than they were, because it genuinely was a maturity issue that made earlier potty training hard for them - they really are ready 'later than used to be the norm 10-15 years ago'. However, the difficult cases probably are just about as difficult as they used to be, but the point where the parent / carer says 'right, we've really got to do it now' and puts in the hard graft is now later.
My DC's pre-school stopped asking children to be dry on admission c. 10 -11 years ago (after DD left). At that time, they admitted every child in the village at around 2.5, very occasionally a couple of months later (DS, I am looking at you), out of nappies, or in nappies if an SEN was involved. Within a year, the majority of the children were starting at 2.5 in nappies, and now I understand the average 'out of nappies' age is around 3.5 at that pre-school.
It isn't that children have changed. It is that different expectations - of potty training being 'easy when a child is ready', of 'waiting for readiness' [both, in some cases, a myth] have allowed the timing to gently slip over time.