When you first release a product/feature, the people that are going to really use it are the "early adopter" crowd. Once they embrace it, then you can spend the capital to roll it out to the later adopters. I'd be willing to bet that Android has far fewer early adopters than iOS.
There's also a major fragmentation problem with Android: Evernote can look at the iPhone 4 sales figures and know that they'll have so many million potential users with the same hardware and OS; with Android they have to compare a ton of different models to see which ones have a usable camera and whether the vendor + carrier are holding up OS releases. If you want to prove an idea that's a lot of extra work.
