The first is that non-technical people don't seem to know what is Android. They know their phone is a Samsung Galaxy, LG G series or a Motorola, but most of them don't know that their phone is an Android or that Google is making the OS. It doesn't help that Google renamed the Android Market to Google Play.
So in the eyes of the everyday man, Google is a better known brand than Android.
The second reason is Fuchsia, which I believe could be Android's successor. I think Google want this OS to be everywhere; on phones, tablets, laptops, servers, IoT devices, etc, and since it will be updated and maintained directly by them, version numbers will be irrelevant for most people. In other words, there won't be a Fuchsia 1, 2, 3. It will be updated constantly like Chrome and probably named something like "Google OS".
So having a payment solution that is named Google Pay make sense when there's a possibility that Android is not Google's future on mobile (and everywhere else).