Some games are games that the Japanese people don't know if Americans will like/find appropriate, so they don't bother exporting them. Americans are very strict about content in games, and that is another reason we sometimes don't get them. It's also possible that they just don't feel like internationally marketing their game. It's a lot of work to do so.
but still i could see alot of games that are exclusive to them be able to work out for use jump ultimate stars battle stadium don
fire emblem ff4 fma brotherhood bomerman blast etc.
Fate/Unlimited codes. I picked this game up because I saw it at a convention and love 2 and 3d fighters. I bought it before they released it as a download on the PSP, and had to mod my PS2 to play it. I was bummed that it wasn't released in the US, but was glad to be able to play it.
I think more games should be released in the US, but I know there are a lot that may not hit as well here.
Well, for games like Super Robot Taisen and Jump Ultimate Stars, licensing is a major issue, so we can't get such great games over here, which is why we import them.