Because currently many apps are Android (and other crap) only. Not being able to install the play store isn't a real problem per se, but simply trying to get APK's will get really annoying really fast. I see myself battling with customer services trying to get a direct link to a APK, hitting brick walls of "What do you mean? Simply download it from the play store"-idiocy again and again.

I guess Jolla users will get first hand experience just how much grip Google has on the Android ecosystem...