Why Android first?

I think it's easy...

Finally defined guidelines (is a new reason for developping Android-First in the current, not the history). Iterations / sprints are faster (you don't need to be accepted by some dominant / censored os guard ) - which for some apps last 6 months or more! (on Android, it's immediatly) Faster user remarks and user recommendations, perhaps a user think's of a feature which adds a lot of value to your product -> more success.

In a later stadium, just convert the fully developed Android version to the iOS version.