Smart cars had to pass US crash test standards and have the appropriate safety equipment. The kei trucks that you can currently import and use are 25+ years old and wouldn't have even passed US standards back then. Your legs are the crumple zone in these things.
I assume that new ones would have a chance, but it'd be expensive for a manufacturer to modify and certify for the US market. Small cars haven't sold well here, and the profit margins are slim.
Maybe with the recent size and price increases in autos here, well see some movement. I'd love a modern Honda kei to go with my element.
Practice?