[spiffytech]

Phone market share has different distributions in different markets. In the US it looks like the iPhone hovers around 50% marketshare. That's pretty significant to ignore.

Then there are the network effects. Many apps have a social component, and if your app isn't on iPhone you'll only get the customers who both use Android and who never want to collaborate with iPhone-owning friends/family/coworkers.

This is of course true for iPhone-only apps, too, which is why folks argue Apple and Google are a duopoly: it's infeasible to succeed in the mobile market without bending to the gatekeepers, and they don't give anyone the market power to force them to change.