It's not all about market share though - there was a developer bandwagon to support Iphones, even in all the years that it was 3rd, 4th, or 5th place, with tiny market share. Even now, with Android nearing 70%, and Iphone around 16%, we see more developer support for Apple (which is sad). Symbian rarely got support, despite being number one until 2011, then still outselling Iphone for another 6 months until the WP switch, and still probably having a larger installed userbase. Not that this means therefore MS will get support - but it isn't simply about market share.
Whilst I'd love to see OpenGL on WP simply because choice and openess is good, it's not clear to me it's the right decision for them. Whilst WP is harmed by it, they are probably looking at the longer term of the Windows platform as a whole. The idea that Direct X would become the dominant API on computers once seemed mad - the API was a mess, OpenGL was open, industry people like Carmack supported OpenGL, and why would you limit yourself to only Windows for no apparent benefit? Yet look what happened.
Also consider that any competent developer can support DirectX and OpenGL. Remember that the far bigger challenge right now is that mobile platforms use entirely different languages (modified-Java for Android, C# for WP, C++/Qt for Symbian, Objective C for Iphone). So if you're rewriting your entire game anyway, the use of a different API seems secondary to that. Ironically, using C# is one of the few ways to get something that runs on Android and IOS (thanks to Mono), which of course is WP's native language. And many games will use 3rd party libraries/engines, so MS will instead focus on getting them to support DirectX, then it's a non-issue.