As I've said before, despite changes along the way the 737 is rooted in a 50-year old design. Baked into that design are some assumptions regarding pilot training, experience and proficiency that are probably no longer valid. Boeing, the FAA, and the airlines all tried to pretend that this wasn't the case as they evolved training according to very mechanical and procedure heavy philosophy and found creative ways to put lower experience pilots in the cockpit. It worked fine until it didn't.