quality. Plus you have to consider the effect of teams having to prepare for and play against stiffer competition more often during the league season. Talent seems more spread out in the PL than LA Liga and Bundesliga with a talent concentration in Barca/Madrid/Munich.
Follow the money.
Interesting that you bring up the PAC 12 comparison. I used to think that it's teams would beat up each other during the conference season and seriously under perform in the bowl games and those factors being related. Sounds familiar.