Because everyone EXCEPT his brainwashed base knows he's full of shit
In 2016 he was a novelty. A lot of independents voted for him for "shits and giggles" and because the other choice was HITLERY THE BUTCHER OF BENGHAZI, RUN!!!111
He won't be a novelty in 2020 and he will not have Hitlery as the opponent. Nor the Comey letter in late October which was about "reopening" of the investigation against Hitlery (which turned out to be a bunch of crap)
People of a very basic level of intelligence are seeing through his bullshit. He'll get some of those ind votes, but not all of the ones he got in 2016, so the cockiness regarding ANOTHER LANDSLIDE IN 2020 (which is hilarious because 2016 wasn't a landslide either) is unwarranted.