That reason is quite simply that the training system in the USA is grossly inadequate to prepare pilots for airline flying. The US system is geared towards producing General Aviation pilots which creates an entirely different set of qualities than something like the EASA MPL, which is designed specifically for airline pilots, does. A well trained MPL with 350 hours and a type rating is a lot more qualified to fly a passenger aircraft than a guy who has 1500 hours towing banners in a Cessna.