I don't think it would be difficult to get the IMV up to compliance with US regulations. If they're selling it in Mexico, it'll be required to have airbags. The hood looks long enough to have engineered in proper crumpling in a crash. Things like backup cameras might require a little bit of retooling, but that's not actually super expensive compared to the other expenses of officially bringing it in: the 25% import tax, a parts and service network, etc.

So it's a business decision not to even try to get it into the U.S., informed by those regulations.

In contrast, something like a kei truck wouldn't be easy to get street legal as a new car in the US: no crumple zone and higher center of gravity are more fundamental safety issues that can't easily be engineered around.