On the contrary - Trump did target him several times (Ohio got lucky and struck oil, Lehman Bros, absentee governor, etc.) but you can't mudwrestle with an opponent who refuses to get into the mud. Rubio, Bush and even Cruz to some extent were naive enough to get onto Trump's terrain and all lost. Kasich gave him the cold shoulder and Trump does not know how to handle someone like that!
My projection all along has been that Kasich will be Trump's final challenger and you will see that in the next couple of months. Tomorrow when I have more time, I will discuss this in more detail!