Why are certain Hd DVD's only available as imports?

Will they all eventually come to the US?

Excuse me if this has been answered before, I didn't see it after searching on here. I'm trying to determine if I should wait for them to come out in the US, or pay the extra money to get them now.

thanks in advance

Most are movies that are distributed by companies supporting Blu-ray in the US, but dist. by a company that supports HD DVD over seas.
J