There are TONS and TONS of small bikes (mopeds, scooters, and small motorcycles) that are not allowed or brought into the US.
Like almost all of the ones shown on the site you listed.
The US government rules and regulations prevent it (or are too expensive for the maker to meet).
Also... the market is too small
A lot of Americans won't buy small bikes (or any kind of two-wheeler)... their ego's are too big to let themselves be seen on something cheap.
Maybe in the future when gas prices keep climbing.... it might change.