According to TechCrunch,
The deal was, as we wrote late last week, in the later stages of negotiation. The two companies had agreed on a price -- around $550 million plus earnouts -- and were working through the final details of the acquisition.
Then something happened that made Yelp reconsider the deal. Over the weekend they notified Google that they were not going to sell, say multiple sources.
There are still no details on what made the deal go sour, although there is speculation that Yelp may have been approached by a potential partner offering some form of a strategic deal that gave them leverage to cancel the Google deal.