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.

Apple Wants to Keep its iOS Developers in the Mac Ecosystem