Because Trump touted his poll successes throughout the primaries. So, poll now to make him look like a failure. A simple strategy. Don't measure opinion, shape it!
Pick the target . . . easy, there's only one in this circumstance.
Freeze it . . . attack him from all sides, media, academia, Congress, Hollywood, donors, Democrats, Republican establishment. Keep up constant pressure from every angle.
Personalize it . . . never talk about the issues he talks about, about all the people thronging his appearances, only talk about the man, how "unfit" he is, what a clown he is, how inarticulate, offensive and classless!
Polarize it . . . caricature him as if he didn't belong, like he was beneath us, you know like Mar-a-Lago Club didn't belong in Palm Beach. Treat him like people who work for a living, not like one of us. Accuse his movement of being violent, outside the mainstream, while having your thugs do actual harm to his supporters.
Make him and them feel all alone, like strangers in their own country.