Of course Apple and developers can choose to ignore Android.
The market within the iOS space is huge and if building an iOS only app using ARKit is more profitable then why not ? It's all about ROI. There isn't an equivalent for ARKit on Android and building one would be costly and time consuming.
How large of a device marketshare could a developer count on at this point from Tango to justify investing heavily in that platform? One thing is for sure, developers can count on a significant share of paying customers if they go with ARKit.
Unless you are targeting a specific audience (for example artists that use the iPad Pro) for most developers it really doesn't make sense to confine yourself to iOS.
The market within the iOS space is huge and if building an iOS only app using ARKit is more profitable then why not ? It's all about ROI. There isn't an equivalent for ARKit on Android and building one would be costly and time consuming.