For iOS, a lot of smaller developers only have one or two test devices, and test mostly or only on the latest OS, because that's how Apple reviews submitted apps, starting right near when a new OS is released. If you don't jump all over the latest major iOS release, then soon your app updates (which fix bugs on the new OS) might get buggier on old OS versions, or not even run on devices with older OS versions. Same thing for new cool apps. Only huge developer corporations have enough staff to commit resource to coding for maybe a couple percent more sales.
Also, without fast adoption, developers would have to spend more on app testing. App compatibility testing (over several hundred different combinations among large numbers of Android device customers) is one of the big things that scares smaller iOS developers away from Android.