37% Of Published Android Apps Were Later Removed, Compared To 24% Of iOS Apps
techcrunch.com
techcrunch.com
1. they just weren't successful enough to be worth supporting them
2. android OS updates broke them
3. services (both web and OS) that the apps were depending on have disappeared
4. some were beta, which were unpublished once the final version was released
I'm sure that's the case with most unpublished apps too.
edit: formatting
That said it's in the consumers interest to discontinue apps which the vendor is no longer supporting, aka abandonware. The annual subscription fee is a nominal cost to professionals (and a tax deduction) and clearly not an apple revenue strategy.
Overall: The advice given is to developers who wish to get out of the app store/ publishing business is to sell or license the rights to their mobile app, creating a win/win situation for both the parting developer and the consumer.