Because 95% of the developers doing 16:9 are Western developers (
http://www.hdtvarcade.com/xboxlist.htm)... then when Eastern developers do try to do 16:9, they don't implement it to the fullest extent.
I said before that it's a minor issue. It certainly wouldn't stop me from getting the game if I was a fan of the series. But overall, there are people that seek out these features in games and Western developers seem to be more on the cutting edge.