What's holding up the Microsoft-Yahoo merger?

The on-again off-again merger talks between Microsoft Corp. (NASDAQ: MSFT) and Yahoo Inc. (NASDAQ: YHOO) are off again, according to The Wall Street Journal.

It isn't clear when the talks, which were first reported by the New York Post, restarted and when they stopped. Still, Microsoft and Yahoo! may join forces on some level. "The two companies may still explore other ways of cooperating," the Journal said. indicating that the two companies understand that they need one another.

Microsoft is betting that its adCenter platform will help it gain traction against Google Inc. (NASDAQ: GOOG). Yahoo! expects big things from Project Panama. Separately, they don't stand much of a chance taking marketshare from the search engine giant. Together, they stand a chance.

Microsoft CEO Steve Ballmer and his counterpart at Yahoo!, Terry Semel, know this very well. Combining the two companies makes sense on many levels.

So why hasn't this happened yet? Do the companies think that joining forces will be an admission that their strategies have failed? Also, even a huge company such as a combined Microsoft-Yahoo! isn't big enough for Ballmer and Semel.

They can't continue on the road that they have traveled. Some change in direction is needed. It's a question of what and when.