Yahoo has been eager to team up with Google for some time now, but has been delayed while regulatory agencies investigate the potential advertising partnership. Investigations from the Justice Department have dragged on for several months, and now, hopefully for Yahoo, those investigations are nearing an end and a final decision is expected soon. Strong voices of opposition from the likes of Microsoft and others have further delayed the move, leaving Yahoo and Google uncertain about whether proceeding would be wise. While the Justice Department can't prevent the deal, they can take the companies to court afterward if they feel the deal violates antitrust laws.
All this time spent waiting can't be good for Yahoo. The company has suffered financially quite a lot the past year, starting Microsoft rescinding its buyout offer which is partially blamed for the massive stock price drop to record lows. Just this week, the ailing company was discovered to be making preparations to lay off over one thousand employees.
Even if a Google deal can save the company, it'll have to come sooner rather than later. As a side note, it is interesting that Microsoft has demanded the deal be halted due to Google owning more than 70% of the search market, and yet them owning over 90% of the OS market hasn't stopped them from selling.