Checkout was a solution looking for a problem.
Their selling point to shoppers was, "trust us with your personal information, not some small website".
They tried to hook merchants by offering free credit card processing for a couple of years, but their API was complex and constantly being revised.
A direct comparison to PayPal is unfair, because Checkout was a Cart (including coupons and shipping options), whereas PayPal is just a payment method.
I think that once G dropped the free processing offer, merchant interest drifted away. I'd be leery to support the new Wallet API's, because of G's track record of retiring projects they lose interest in.