Because it is creating a checks and balances for the president within the checks and balances of the current government. It would require such a constitutional change, it would require more actions than just a super majority as we believe it would.

Edit: sidebar. Each round of eliminations would present new questions, each candidate would submit. Some would be "illusion" of current or future events but were really past events. This prompting a past president or leader (Congress member, senator etc). To bring up and discuss what the false narrative was, showing what the realism had been. Then giving evaluation of how they responded at the time, and how it went right, wrong, and what could have gone better if done differently. Thus educating both the population and the candidates in doing so. Basically, the first reality TV program worth turning in for or watching brought to everyone via national TV/internet services for free.. and using the ad segments to pay for the costs associated with the applicants. Doing away with campaign fees.

Side bar 2: Yes that means if you serve the nation as a president/ congressperson/ senator / or ambassador you may be called upon to serve your country for a lecture... But that should be fair, as we pay the secret service to protect you for life. A lecture twice after you retire won't kill you. (Shit even Carter would have loved to do this 2 years ago because he wanted to believed in this country). And I would have called him to old.. but with cards and his choosing, I would have been greatful to hear him give peanuts to pinenuts