Because he lies, a lot?

And: constantly misleads people with half-truths and lies; sacrifices America's long term strategic interests for short-term gains; diminishes the office of the presidency by picking fights with actresses, reporters, musicians, atheletes...etc; lacks a basic understanding of the facts; tries to start a trade war on stupid, outdated economic assumptions that no longer applies; empowers racists and white nationalists; fails to keep his white house in functional order and fails to keep white house staff employeed on a substaniable basis...etc?