That's also because deploying an APK on a test device (or several devices) is way easier than deploying an IPA on an iOS device.
As someone relatively new to iOS this past month or so, I was shocked by the number of hoops I have to jump through just to take a piece of code I've written and put it on a device that I own and is sitting right next to me[0].
Coming from Android development, I never thought twice about this - it's easy to push the APK to my phone as part of the build process, and I don't even have to plug my device in to do so.
[0] Including shelling out $99 for a certificate that gives me the permission to do so.