Thanks. The reasoning wasn't terribly sound. (Android and Chrome OS overlap regardless of the target hardware).
But from a branding perspective it makes sense to distance a immature product from a widely popular product. (Obviously that wasn't the idea though since Android Auto, Android TV, Android Wear)
I can also see why they wouldn't want to kill off Chrome OS since it allows them to experiment with a diverging set of strengths and features that Android might not be conducive toward. Those strengths could be leveraged in their other products especially Android.