Well I think the problem is the target audience in Japan v. the target audience in America.
My friends and even a lot of people on other forums and ircs think that iDOLM@STER is a game for GIRLS.
It has a lot of traditional elements of girlish games I suppose. Dress up, watching the performances, gorgeous men, etc etc.
Obviously in Japan the target audience is men...but if they tried to market it to men in America (I don't wanna play stereotypes here, but by men I'm thinking the stereotypical xbox live addicts) it would come off real pedo-ish and not all that appealing...because I think here it definetly looks like a game for girls.
Now, if the game was targeted towards girls, it might be really successful!
"Look you can dress up your character however you want and watch her sing and dance! Isn't that cute?"
This. I have a (male) friend who wants almost nothing to do with the girls (except for Makoto and Hibiki) but would prefer playing Jupiter because they look badass
unless they're skipping to Koi wo Hajimeyou 
.
Obviously, there's nothing wrong with a guy liking iM@S, but the vast majority of American men with their videogames and manly I-must-kill-things-with-my-manly-guns-in-war-time man-attitudes aren't going to go for dressing up a cute 14-year old and her buddies in order to perform a cutesy song.
Okay, okay. It was an extreme exaggeration. Don't chew me out for it, manly-men.However, if you translate the game, I'm sure it'll get itself a following, besides us on the forum. Because it's kind of a strategy game, and kind of a rhythm game. And I'm sure when you get to people that normally play games that have heavy conversation/relationship aspects (a la Persona 3/4, and Catherine), they'd be glad to pick up iM@S and give it a try.
That being said, Persona 3, Persona 4, and Catherine are all rated M. And I'm sure there are games with that sort of thing going on that -aren't- rated M, I just haven't played much more than those, if any.
It just wouldn't be the next MW3.
I'm afraid of what would happen if they market it for girls. It's more likely they'd dub it over and do a bad job with it in general, I think, and less likely that they'd leave in the Japanese voice track and use more literal translations. Of course, that's a really general assumption and I shouldn't
assume it would be that way, but the possibility frightens me. XD