Aside from import tax issues, it also has a lot to do with meeting our safety and emission standards. Not that they wouldn't meet the emission standards as they are, probably just lots more hoops to go through and not worth it for the manufacturers.
This would be my guess. I haven't found any hard numbers, but I seem to recall reading years ago that every engine / powertrain combination in each vehicle is required to undergo vigorous, extensive (and expensive) emissions and safety certification processes before they can be offered for sale in the US.
Given that scenario, it stands to reason that manufacturers would only offer models that their studies indicate would sell in sufficient numbers to justify the certification costs.
