This is just a guess, but I think for the most part it was a reaction to the Colgan accident captain's method of reaching the level of airline employment. He had hours and hours logged wherein apparently not the appropriate learning had occurred. He failed a bunch of primary training, and then instead of going to learn something for a thousand hours that may have been useful someday, he saw an easier route... paid for a "job" building "valuable" SIC time and proceeded to learn almost nothing that might have helped him know how to pay attention at critical moments.

Hours are a crude way to judge a pilot, but they are one of the few ways available, so here we are with 1500 needed. It is trying to prevent the above pathway of "learning" and "skill development" from ever happening quite so easily again. If you have a giant pile of money, it is probably still possible to learn nothing and have 1500 hours. The point is, most people don't, and those that do... why would they need to become airline pilots? They'll take care of themselves in a Cirrus/Bonanza/Citation someday.