I'm willing to bet the only reason they did that is because they already had beats customers on android.
The whole point of these APIs is to let developers make a better apps on iOS so people want an iOS device. If they port any of them to android all they're doing is making it easier for people to leave their platform.
Sure that's the goal but in practice I don't think it's working. People use cross platform tools like Unity or React Native or OpenCV because they know they have to support Android and can't kick that can a year or two down the road like they used to.
So then use Unity with ARKit and wait for an equivalent android AR library. I’ve looked into ARKit myself already - it is dead simple to get started. And so much data is already provided, including scale and lighting estimates.
Yes, you lose out on android for now, but in the mean time you support millions of iOS devices in a category that will be getting a ton of media attention in the next year.
The whole point of these APIs is to let developers make a better apps on iOS so people want an iOS device. If they port any of them to android all they're doing is making it easier for people to leave their platform.