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.
Because I am not a child, I vote based on policy issues and not the politics of personality. Trump is a scumbag and if he died via a coronary, I'd celebrate his death but I'm opposed to him because I think his agenda is repugnant, disgusting, reprehensible etc.
To repeat my previous post: most posters are pessimistic about Trump because of his cabinet appointments, his brash undiplomatic and dangerous behavior on twitter and the agenda of the Republican Congress. He's a scumbag, total filth, one of the worst people to ever walk the Earth and so on. He's also insane. However, that's not why this forum is, for the most part, anti-Trump.