I can't say for sure what the reason is but i have a pretty good idea of why this happens. I think people are simplifying it to much by thinking it is only 1 reason.
I think the number 1 reason is simply time, the Japanese are extremely busy.
Another reason is virtually every wii/ps3/360 owner probably owns a DS/PSP. While some people deny it the DS/PSP is competing for our time/money.
currently playing: Skyward Sword, Mario Sunshine, Xenoblade Chronicles X