That's because the RNC will use dirty tricks to stop Trump from getting the nomination. When was the last time a party's own front-runner was targeted for defeat? If you're the front-runner and expected to win, and your own party is stopping you, wouldn't you go 3rd party?
Trump supporters when someone else threatens to go 3rd Party
No skin off my back if anyone else runs 3rd party. Trump supporters aren't going anywhere, that's the thing the GOPe just doesn't realize. And it'll actually eat into Hillary's support.
When was the last time a party’s own front-runner was targeted for defeat?
Ronald Reagan.