Because Petco, LA and SF are the same as Hou, Chi, Cin right? If I had to pick the easiest division in baseball to pitch in I'd easily pick the NL West, it has a serious effect on how much belief I have in their pitching stats. Those are only 3 of the parks in the division. Coors and Chase are hitters parks. Coors is a hitters park because of how breaking stuff flows in the park and Cain has figured out how to pitch there. The Diamondbacks have had a lot of mediocre offenses over the past 4 years. It is still the easiest division in baseball to pitch in. He still has a career 4.11 ERA in Coors though. 4.22 in PHI, 4.13 in ARI, 5.40 in HOU, 5.56 in MIL. In general he has been a 4+ ERA guy in parks that don't hold his HR in and an ace in parks that do hold them in. If the Brewers had drafted Cain he'd be a forgettable 4ish ERA guy who was underrated. Instead he is an 'ace'.