My two cents:

The Apple app store is successful because it's a toy store. People like browsing and finding cool stuff to play with.

For WordPress themes and plugins, however, 99% of the time, users are looking for something that fills a specific need. The "browsing" experience doesn't really exist (again: for most people, not all). You get your site working, and there's no need to peruse a store every so often.

All that said, I think there still could be a place for the WP App Store. It's a lovely site and plugin, and I'm interested to see how it evolves.