Because they still have their social/competitive roots in arcades. Also, Japan's superior nework infrastructure, network speed and tiny relative land mass make online MP work much better for them. In Japan, XBL is the best thing since sliced bread. The mediocre netcode slapped on most games works WAYYYY better there than here.
This pretty much, also the Japanese never really got into the VS series, they've always hated it, which explains why MVC3's online is bad even compared to other fighters.
One thing I noticed last night, I can now agree with this sorta....I've seen some people say Gouki (Akuma) is too strong, I've seen some people say Phoenix is too weak. All I know is I did 1 combo, canceled into super fireball (no X-Factor) with him on Phoenix and it killed her. (this is with Phoenix @ full health) It was a simplistic combo too....nothing crazy, no OTG's, no assists. So either Akuma is too strong or Phoenix is weak as hell. (checks SRK wiki) Yeah, it's both. According to their wiki, Akuma is low health, high damage, and Phoenix has about half the health of the weakest characters in the game. [note to self - don't bother with Phoenix]
A lot of people can do that to Phoenix, as for Akuma, he's a mixed bag. In every game* Akuma has been in he's always had crap health but massive damage output. He's quite strong in this game with a lot of ways to put out insane damage, demon flip mix ups (which was always key to him), OTG supers (huge advantage), and insane pressure. What makes him really scary though is that his level 3 super, is damn near an instant counter to anything and does insane damage, with xfactor it will kill just about anybody.
It's a bit to late to declare anything on most characters, all that is agreed on is that Sent is, once again, broken.
*not true in the original Super Turbo which is why he is banned.