Because it's difficult to understand why anyone with enough intelligence to operate a computer would support him.
I'm not saying Trump supporters are idiots, but I have yet took figure out how one would support him otherwise.
I would never vote for him. But it's hard to not have sympathy for someone you see getting attacked and slandered for saying something you agree with. Political rhetoric like "supporters are X are idiots" creates polarization. People who were on the fence before become stronger supporters of X, because they feel they are under attack and need to stand up for what they believe.
Trump is a master of this tactic. He says things he knows lots of people believe, but that are very controversial. This gets him tons more media attention than other candidates.