Pre solo checks were usually conducted at up to a 100 kts, when one knew naff all, and had no long term learned habits or skill set.
After perhaps years and maybe 1000s of hours doing things at 180-240 kts, one develops a well honed skill set and mind set. Bump the speed up to 450+ kts, triple the weight and thus inertia, and replace rotating speed brakes (props) with small, fairly ineffective boards (spoilers), and it takes some time to modify the skill set and mind set.
It's not a criticism it's an observation. It can often take more time and when a company has a lot of folk that take a bit more time, then budgets & time lines can blow out.
train driver, are you saying that none struggled in the beginning? I've not seen anything in this thread suggesting that they continue to struggle. I certainly know my first jet endorsement was more challenging because of the speed etc, than had it been another turbo-prop.