I've often wondered if you took Sandy Koufax at his prime and disguised him as a 17 year old how long would it take one of the best pitchers in history in his prime to make it through the system to the big leagues? Dwight Gooden went from 18 year old Class A straight to the big leagues and put up the same terrific numbers in the majors as he did in Class A. I'm not saying Thorpe is Koufax or Gooden but given that all the attributes Seth listed is where he sits right now, why is it a fast track to spend 4 more years in the minors before getting a shot at the bigs?
Ideally teams want to time a player's MLB arrival to coincide with the start of their 6-7 most productive years. Teams simply won't call a 19-20 year old up and let him get knocked around a little bit for 2-3 years at the MLB level. Those become lost years of productive contractual control as the guy gets expensive sooner.
If the Twins keep Thorpe on the farm, they could control him until he's 30. If they push him up to the Big Leagues at 19, even if he turns out to be another Koufax, they could lose him just when he's hitting his prime.
As someone who has been watching MLB ball since the Koufax days, I would also offer an explanation that is nothing more than opinion: I think hitters are just that much better today.
It's not that pitching hasn't evolved and improved over the last 50 years, too, but it seems to me that hitters are better athletes, using more advanced training methods, work full time almost 12 months a year at their craft and apply more advanced analytical information to their craft. There have also been rule changes that have strongly favored the offense.
Sure, there were hitters in every era that could be successful today, too. But your average (and even below average) hitters today are, I believe, much better than they were in prior eras.
Pitching has improved, as well, but it doesn't seem to me like that aspect has improved as much as hitting has.