This was Marco and other iOS developers' mistake. If they wait too long to come to Android, there will be established competitors on the platform, and instead of being considered a market leader, they might even have to play catch-up with those already established on Android.
This is the same reason why even Microsoft has decided to make Office for iOS and Android. They can't just ignore 700 million users (and soon to be much bigger) just because they'd want people to use Windows instead. Windows is a big cash cow for Microsoft, but so is Office (about as big in revenue), and they can't risk losing the Office market to others, and not getting to decide the future format for docs, and other benefits they now get from being the absolute Office leader.