Because the pitching in AAA absolutely SUCKS. Seems like most teams anymore put their best pitching prospects in AA, and when they're ready, promote them straight to the bigs. Not an absolute rule, of course, but it's happening much more frequently than in the past. The net effect is that the quality of AAA pitching is significantly diluted. Look at the Cubs recent young pitching promotions as one example. For all practical purposes, Sean Marshall, Carlos Marmol, and Billy Petrick made the jump straight from AA. Only Rich Hill and Mike Wuertz saw significant AAA time in the minors. And look at the Cubs minor league rosters right now. Until Sean Gallagher's recent cup of coffee in Chicago, for which he earned a roundabout promotion from Knoxville to Des Moines, the Iowa staff "featured" the likes of Les Walrond, John Webb, Ryan O'Malley, JR Mathes and Ben Howard; yikes. Meanwhile, AA has had Sean Gallagher, Donnie Veal, Billy Petrick and Mark Holliman, amongst others.