They're more practical in Europe and the UK, where crazy mothers driving their single child to soccer practice with a cooler of Capri Suns in a Suburban that could easily seat three border-hopping families are not as much of a problem.
Europe has negative population growth. Europe has shorter distances and more expensive gas. Europe has smaller average vehicles (due to smaller and windier roads). It's natural to have a vehicle like this there because the driving culture evolved to it.
In the US of today, it's like putting webbed feet on a donkey unless and until general driving habits and the driving culture in the US change dramatically.
I can maybe see these in an urban environment where maybe you don't drive far and use surface streets (i.e. the same kind of areas where mopeds are practical) but once on the highway I can't see this being safe or practical, and especially since average family size in the US makes this a useless toy (especially with increasing migration to the US).
Edited to avoid HELL of negative cans because Karma can ban you now.