Russell Ivanovic of Australia's Shifty Jelly (developer of my favourite weather app for iOS and OSX) talk about why they developed an Android version of their Pocket Casts 4 app before an iOS version.
There's no reason not to launch on Android first or iOS first in 2013. Both are massively viable platforms full of users who want to pay for great apps.
This is a mature and reasonable decision for an app developer. Focusing limited resources is critical, and it makes a lot of sense.
I think it also makes sense based on competition. As Russell stated, the Android version has been a bigger seller and more profitable product for them, and there is no native app in the class. I also suspect that relatively few developers are developing "Android first" (for now), so there is an opportunity for them.
I am a committed iOS user, and will be for the foreseeable future. But I've made platform shifts in the past, and can say that what makes sense for everyone - developers and users - is to choose the platform that provides you with the most value.