Google isn't that great at marketing. See: Google Buzz, Google+ (sucks), Google Video, etc. The only thing they had going for them is Gmail, Android (which they bought), Search, and YouTube-
They bought Youtube as well.
I would venture a guess that the vast majority of iPhone owners don't know that Google Wallet is available for iPhone. Granted, it wouldn't do NFC... But it was an option.
Part of the problem with Google Wallet is that your card isn't actually used at the vendor.. Google pays them through Mastercard. So some reward programs that rely on you buying things with your card won't work.
Google also had lots of issues with the fact that carriers blocked the adoption of Google Wallet on the handsets on their network.. In one article I read, a journalist went to test Apple Pay at a store that he had tested Google Wallet at two years before... And the clerk said that no one had used Google Wallet since he had!
Also, back in the day, the Nexus was the only phone that supported Wallet. That changed - eventually - but I think it's safe to say that the ship had sailed by then.
The funny part is that Apple Pay might actually revitalize (interest in) Wallet.. So perhaps Google has a second chance to market the thing correctly.