I also suspect it's because the demographic of Mac users are generally richer and more likely to pay for software
In fact, many developers just don't bother any more and build only for iOS. The users are generally willing to pay for quality apps, and that's sadly rarely the case with Android.
During that time people got used to alternate sources for .apks and they didn't move back. Most likely they converted other people too.
I would argue that iOS is easier to develop for if you're prepared to learn Apple's dev tools. The hardware available is locked down so you can more easily design for all cases, and there's a good bunch of core frameworks covering a lot of functionality