Focus is a big one, as nate mentions below.

The second is that when you've spend this much time working on a project, watching it die because you no longer have the time/focus to work on it is painful.

I've been in this situation several times, and being able to hand a project off to someone who will take decent care of it is far better than seeing it go under. There is a personal connection to projects like this, to the point where the time invested in selling it might not make sense to an outside observer.