Well, as discussed in this article, this probably because Groupon didn't think the deal would have been approved and gone through. But at least Groupon saw some money: 6 billion, knowing it at lease worths that much now. Now it started to think about raising money to make the company value at just about or above 6 billion. The question is: why google would pay 6 billion for a company makes about $500,000 a year? All the so called "deals" are mostly spa or beauty services, or restaurants. There isn't review available of the business providing the service. I see Yelp.com has a much better business model: they list all (almost) the business in the area, providing with tips, reviews, and business can claim the its yelp and announces special deals on Yelp.
It's not that Groupon management was stupid not to take the offer from Google, but they didn't think that the SEC would approve the offer, so they would see the money anyway. They're all smart people, but Yelp will be a winner here.