Okay, so let's see.
Metroid 1 - 1.04M - the 39th best selling game on NES in Japan.
Super Metroid - 0.71M - the 34th best selling game on SNES in Japan.
MP - 0.12M the 29th best selling game on GC in Japan
MP2 - 0.07M - the 31st best selling GC game in Japan
MP3 - 0.07M - so far the 13th best selling Wii game in Japan.
So my guess is as follows.
It has nothing to do with Metroid specifically.
There are many less big sellers in Japan than before. Japaneese aren't buying different games anymore, they're buying the same ones. Therefore, Metroid has now come in the "not quite good enough" basket for them, along with many other games.
http://www.vgchartz.com/games/userreviewdisp.php?id=261
That is VGChartz LONGEST review. And it's NOT Cute Kitten DS
