I can't believe nobody has come up with the correct answer.
John Kasich does not want to serve in a federal government that is led by Donald Trump.
Yet he’s fine being in a party led by him
Please, he's a term-limited governor. You expect him to switch parties?
Zell Miller didn't agree with or like pretty much anyone in the Democratic Party at the national level for at least the last 20 years of his life, yet he remained a Democrat, in his own words, "until the day I die".
Kasich's relationship with his own party isn't nearly that adversarial. I can't imagine him giving the keynote address at the DNC.