Well, one of the issues I've always had with new cars for K.I.T.T., is the visability factor.
Michael could sit in K.I.T.T. across the street and not draw attention from the bad guys -- the car sort of blended in.
But in today's world, that model/look, is fast becoming out-dated and now any bad guy might take a second look ("Hey, look at that old calssic!").
So many of these proposed K.I.T.T. ideas are overly fancy, shiney, aerodynamic like they are going to be reaching moch speeds, expensive looking, and would stand out like a sore thumb even in today's world of newer cars.
(and a lot of them are too low to the ground; a filming model would have to be jerked up real high, making it look weird, to do turbo boost landing shots; in fact, that might have to happen anyway)
It's the belivability factor.
And when things appeared to get better, they got worse, and everyday was a day to cry.