The probable answer: JP casuals are not in the same solar system as EN casuals in terms of how casual they are.

I like to think of myself as casual, but I enjoy a little complexity in my day-to-day stomping. Viper was a welcome change to the usual brain-dead slowness of DPS classes.

I don't browse the JP forums, but I've heard from those who do that they were absolutely foaming at the mouth that Viper was not friendly enough to casuals.

And, of course, they get priority when it comes to feedback. Because JP game.