Other than the time factor for volunteers, probably one of the reasons that something more formal hasn't been implemented is that some plugin developers would be more inclined to host the plugin themselves.

We host some of our own. For example, I have a login widget which I've maintained since WPMU 1.2. When I wrote it there wasn't a repository. Now there are 3 or 4 other slightly different login widgets in the repository. So, I haven't rushed to add it.