Ben Brooks has a fantastic addition to my Mac App Store piece, stating the store was launched not primarily for the users but for the current pool of iOS developers:
My guess is that if they truly did this as a simplification of the OS they would have waited until 10.7 — giving everyone a clean breaking point for making the transition.
Apple reached a fork in the road: they could have gambled on iOS developers being willing to develop for the Mac when they release 10.7; instead they chose a safer path of launching now, at the very moment interest in such a distribution channel was at its peak.
And note that most of the new apps in the Mac App Store were ported iOS games.