Add to that the fact that Yahoo had a recent serial history of hiring then firing CEOs before they brought in Mayer, and the pool of potential people to fill the spot shrinks again.
Yahoo had no pathway to success and was likely to fail spectacularly - especially given the way the board was behaving - which signified to any potential hire that it might be their last job. The pool of potential CEOs shrinks again.
Of course the incoming CEO would negotiate a well-beyond-market severance package. They shouldn't even want to hire someone who didn't have the foresight to do so.