There are several reasons that not all models are available here. The main reason is money, for the most part there would have to be a large enough market to make it cost effective to sell them here. Expensive sports cars would be a very small market, adding more marquees and models to this already crowded field would likely end up with the new companies failing here, and probably some of the aleady established players leaveing the market. There just isn't that big of a pie to slice up. I wish there were more peole willing to pay 40K plus ofr a two seater but there aren't.
The second reason is regulations, diesels are screwed based on US emmisions requirements, there are other models that would require signifigant redesign for US safety requirements. Typically it is not small things like yellow/side marker lamps that prevent the manufacturer from importing them here, as those things would be very easy and cheap to install. It is not easy to reduce NOX emmisions in a diesel or redesign a unibody to conform to safety standards, without changing the nature of the car.
I am not saying I agree with the laws and regulations, just saying that it is generally not minor little things that keep these vehicles out.