EDIT: to be clear, currently on Android any app can handle NFC payments, not just Google Pay. Banks could easily force people to user their apps, but that's not happening.
Then Apple Pay launched here, contactless adoption on iOS became triple of Android's (despite Android having more than double the phones), and a few months later Google Pay launched in response with their better implementation. I'm guessing it was a combined "in response to Apple" from Google and "maybe this will increase adoption" from banks.
NFC payments were possible through the banking apps on Android for years before Apple Pay launched in my country. Note that contactless payment had also been around here for years and pretty much all payment terminals supported it. Technically, everything was in place for it to be adopted, yet no one used it. Only after introduction of Apple Pay did it take off. Google Pay support followed the success of Apple Pay.