Because people are angry and frustrated and he's the one who's speaking to their frustrations. Even if they don't like or agree with a lot of what he's doing and saying, at least he's making noises about doing something to try and deal with their issues instead of doing nothing. Granted, Clinton was doing that as well and had detailed plans about how to implement these things, but that information got lost in the reporting about how she supported transgendered bathrooms and was about to be shot by a firing squad for treason because she sent classified emails to terrorists.
This. Trump won because he pushed the message "Make America Great Again". Hillary lost because she pushed the message "I'm with her", instead her being with us. Trump preached about what he, and only he, could do for the people. Hillary came off as what the people could do for her. This may turn out to be completely backwards from the truth of both candidates, but messaging matters, and Hillary's self-entitled and bought out (taking money from wall street) image killed her politically, probably for good.
And lets not forget the impact of the regressives on this, calling everybody racist, sexist, whatever all the time no matter what. A lot of people are sick of it, and that includes actual racists who were charged up and lashed out.