1) Because they're japanese games
2) Because the Xone is a disaster in japan. It's only at 50k units now, will probably do 100k at the end, while the X360 did 1.7 M.
3) Japaneses games on 360 flopped hard.
Some dev nearly crashed because of this (namco with tales of vesperia), and they really, really don't want to try something like that anymore.
4) the public for japaneses games is NOT on Xbox consoles. that's why jap games flopped on 360.
Xbox is more american oriented, so Fps/Tps/racing/occidental-rpg.