Because POTUS is at any one time the most criticized person in the US:

I imagine Trump not being able to focus in the Oval office, desperately trying to defend himself and/or attacking others via the IRS, GA, DoJ, Military, CIA, NSA, Homeland Security, LOL.  He would be an unmitigated disaster.  He lacks the 20 inch Teflon coating that is a must for anyone who seeks that office.

Putin and other world leaders that oppose the US leadership would be able to play him like a violin, IMO