Because he's demonstrated that he is not qualified to lead? Every day since the election, he has shown more and more how unqualified he is to lead.
So you would rather have a woman who, "qualified" by virtue of holding different offices, nevertheless had no accomplishments of her own to speak of; who took advantage of others for her own purposes; and who is known to be a heartless, repulsive woman concerned only about her own advancement? I would rather have a political rookie win the White House than someone marred by scandal, who is also untrustworthy and greedy.
First off, experience counts when you are running for President and I think would rather have someone who has held office before as President over a complete novice. Second, Clinton has a substantial amount of accomplishments that she achieved on her own, whereas Trump was given a check by his father. If you think Trump isn't out for himself, then you are not well informed.
If we want to talk about heartless and repulsive, just read about Trump's past antics. And untrustworthy and greedy describe Trump's entire career. But at the end of the day, it's pointless making these comparisons, because the election is over and Trump is President.