Because that's the going rate for CEOs. Any of them could go to some private corporation and make just as much, if not more. So, nonprofits have to pay competitively or only afford CEOs willing to take the pay cut.

Which is all really terrible because no CEO is worth that much. They only get paid that much because big corporations have absurd amounts of money, and the people in charge of setting salaries have no restriction on paying themselves that much.