Uh, because he's a criminal, among other things? How about because he's an idiot? How about because he's an embarrassment to our whole notion of competent governance? How about because he's an overweight goon with an over-inflated ego and little to justify it?
And yes, I'm familiar with the 9 charges out of 10 on which he was convicted in Federal Court in Ohio. It's fascinating to me that so many people defend this buffoon just because he's occasionally proven useful to the Republicans. If he's the best we can do for an ally, this is a fight better conducted alone.