That's a good point, but it's also a hen and egg problem. If the platform isn't that widespread you might be hesitant to support it. (assuming it's not actually super easy to port over Android apps as suggested).

While Windows phone definitely was a very different platform from both IOS and Android, it was still amazing how shoddy some major players' apps ran on it. Stability, performance and features were often lacking. The tools, documentation and lack of sample code was definitely to blame, but even then you still need to learn a new platform.

App developers have already trouble "jumping" onto the latest version of the one or two mobile platforms they support, I doubt there would be much enthusiasm to add another one to the bunch.

Indie developers often support only one platform because any more than that is practically infeasible with a small team (n<3) to do if you have a modicum of respect for your clients.

You've missed Amazon out here, who have a vested interest in keeping the non-Google controlled Android competitor they have as the only one.

Every other app also has competition, bugs and features to keep them busy with their iOS and Android apps