Im not 100% but I remember hearing that its the SR20 engine not able too meet the test requirements thats why the 240sx has the KA engine.Like I said not 100% its just what I had heard,hopefully Nicely will be able to expand on this question.
I believe its more to do with the entire car. There are plenty of SR20DET conversions in the US. Paul2x would be the better one to ask, as he has a road legal US import