Earlier quoted context omitted.
Because we feel like we can offer better service by specialising, since we can test the apps ourselves, etc. We want to give every app a considerable amount of attention so the final result isn't just a translation that "makes sense," but that will be truly appreciated by users in Japan.
You should be specializing in translation, regardless of the format you work for. And I'm pretty sure you can find a cheap Android phone around to test if things work right.
My partner already does "specialise" in translation. She does freelancing for a living. I simply had the theory that it'd be more effective to niche down, and I chose to target iOS because: I'm a user of iOS, I have some iOS development experience, and the App Store is growing in Japan. We're not basing our livelihoods on the decision. It's just something we're testing.