Well, for starters, apps would be 43% more expensive to allow the devs to make the same profits after Apple takes a 30% cut, for one.
$1000 software available for $1430, but hooray, you don't have to use Google to find it!
Secondly, if it's not compulsory to use, then it's not a one-stop repository for apps that you want it to be. You would still have to hunt it down on Google. If it were compulsory for all developers to use, then you have the problem of the 43% price hike for consumers, as well as the whole approval thing, as Apple now have their reputation on the line for these apps.
Try Versiontracker