I think it's because you have a million "fact checkers" trying to prove that everything he says and does is wrong. By doing so they're forced to investigate everything he says in depth, and over time, they see that there is more than a shred of truth to what he says. They realize he is pointing out real issues and even more so, ask themselves why the media never brought this up before. Some are just paid shills, radical SJWS, and astroturfers that won't ever change their mind.