I suspect Google was looking at the possibility of modifying it to fit in with their advertising business. I can envision several ways that could have worked out.
It is usually easier to modify working code than to write something from scratch, if my suspicion is right, I expect Google will launch its own Groupon-like service integrated into its other offerings eventually.
Google isn't infallible, they are mortal. I think most likely they were willing to overpay because they could afford it and are desperate to get into "social".
Also, Groupon is a ponzi scheme because they use new investor money to pay old investors--not because they don't have a moat, which they dont-- no patents, 8000 employees and how do you scale with that many employees, a list of merchants and user emails.
At best they end up looking more like Avon.