Well Zerbe is well into his 60's and doesn't want to be running a business on a day-to-day basis - again primary reason he hire Khosla.
I guarantee he has had offers on the business - but they were probably well below what he wants (and frankly what it's worth, as people will low ball because there isn't any real revenue yet).
All depends on if Zerbe is ready to take far less than he was expecting to get within the next few years. Now the board has to go into a search for a person who they probably don't have first hand experience working with (like they did with Khosla).
Any hint though of a buyout I think will push it at least a little higher.
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