Because who would trust this man to lead the greatest ARMED FORCES of the world? Everything about this man is a failure masqueraded to look like a success. All of his business ventures have failed. He's lost instead of gained from his father's wealth.
He has stupid opinions on a lot of stuff, sure, but the worst part is that he has not the slightest idea about politics. He doesn't know what he's doing.
If he becomes president it would be an absolute mess, cause he would be overstained.