They incentivize people porting to iOS by being a great platform with tons of customers, many of which are willing to pay. That's why so many apps launch on iOS first (much to my dismay as a diehard Android user).

Why Google hasn't done this, I don't know. Seems like it is in their best interest and something they're already doing internally. They use the shared web/Android/iOS code concept for Inbox (but not HTML/JS)[1] and probably other projects.

[1] http://arstechnica.com/information-technology/2014/11/how-go...