He likes being a free-wheeling colorful character who says whatever he feels like at the moment regardless of information or tact. Many jobs would be fine for that, but not this one. This one requires some prep, some discipline, and some organization. Something is going on with Donald Trump.
Trump has been right about many issues, but let's just call this a big mistake and bring on Pence, please. He is the real deal and Trump seems to be begging to be thrown off the ticket. With too many, Clinton is a default vote. Almost nobody except grey hair feminists on the Upper West Side are eager to vote for her. Because, vagina.