There are a lot of reasons why people don't use Apportable, most of them don't have anything to do with Apportable's actual technology. Those reasons include: not willing to pay for it, and the fact that iOS design patterns (human interface guidelines) are completely different from Android design patterns. You cannot simply port over the exact same buttons and table views from iOS. Apportable will port those over just fine, but you basically need to redesign the entire app to be different on Android to have something that isn't jarring. At which point you may as well write everything in Java.

To do that, they'd need developer cooperation. Developers are notoriously unwilling to deal with any app stores but the Apple and Google ones; look at the state of the Amazon and particularly the Samsung ones.