It's not the number of Apple users that matter. There are many more Android users world wide. It's the fact that Apple users spend more, and more often than users on other platforms.
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.
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.