It's not just pull ups. Modern disposable nappies have a role to play, yes, but perhaps more significant is that mother's aren't at home as much as they used to be. And let's face it, 90%+ of people doing potty training are the mother's.
Part of the problem is the amount of women entering full time employment rise and rises as we get to now.
In 1950s most mothers would be at home with kids all day, perhaps some working part time, and fewer still working full time. So, it was easier to facilitate potty training at home? Mother's don't perhaps have the time to train earlier, as often a 20 month old is in full time childcare, and perhaps being brought home by mum at 5:30, and then it's dinner, bath, bed,.asleep at 7. Wake up at 7 and back to nursery for 8:30am the next day.
So naturally, women will be "forced" to wait until perhaps nearer 3, because by waiting for another year, they child can be trained in a week or less perhaps, as they're easily ready. But, at under 2, they're more likely to take a bit longer to grasp the concept, bae the ability to get undressed in time etc so disposables give most working mothers a convenient "delaying" method, which fits in to the modern world. Just like they don't have time to hand wash terry nappies etc the washing machine saved time, which is turn enables women to work etc etc. it all feeds each other and isn't as simple as "modern nappies make mums lazy".
There's a lot of blame placed on women for "delaying" potty training, a lot of shame, a lot of implied laziness. I really do wish women would stop beating other women for not doing things exactly the same way.
ifs.org.uk/sites/default/files/output_url_files/BN234.pdf