I've got two theories :
The Republican-controlled U.S. Congress has been unable (or unwilling) to oppose the dictates of a power-grabbing President. This spells the end of the voters’ confidence in Republican promises, opening the door for a new second party (led by Donald Trump?) and relegating the weak and ineffective Republicans to third party status.
Trump is having more fun and attention than he ever dreamed of having, but if he were elected President it would be a disaster to his financial empire and his career as an incredible businessman. As President he’d have to work around the clock for peanuts. The millions that he could grab as President that lure his opponents, Democrat and Republican alike would be (for him) just chump change, so although he’s enjoying the campaign, he really doesn’t want to win the Presidency.