Kasich is quietly mounting a challenge to the President in 2020.
Besides, the President does not suffer fools gladly and Kasich has earned all the "trashing" that comes his way.
It comes off as Trump and his supporters being unable to let go of the 201t6 election. They aren't satisfied with winning the last election. They have to continue beating the loser(s) down to a pulp.
Again, it's a waste of time and effort. Should Kasich be a candidate in 2020, Trump can speak out against him then. But why go after Kasich now? Focus on the present. We have some real, more immediate enemies to deal with.