On this official Google Blog you can read an update on the Google Work on Payments. It explains that at first Google needed a payment infrastructure for itself to pay out for the Ads. That used to be all checque based, but over the years Google improved towards ACH and bank-links. Having done all that, it's a smaller step to using the payments connection for users themselves as well (see an example of the purchase and payment process here on Scot Wingo's E-bay blog).
A perfect example of how new forms payments always pop-up on the back of some other primary activity:
- buying/selling via e-Bay (Paypal)
- making phone calls (m-Payments)
- travelling (travelers cheque/ credit-card).
It wouldn't pay for anyone to develop a Google-payment product from scratch. But it does if very similar technology and the user base is already in place.