It does make me wonder though. IIDX had a pretty steep learning curve that didn't make it particularly conducive to the US general crowd. That, and the fact that every time I see IIDX at an anime con or arcade (and that's the only time I ever see it), it's always surrounded by a small handful of elitists who either mock the rookies or hog the machine. That's just plain old unfriendly. I used to run the video game room at our local con. One year, three people on the forums were clamoring for IIDX, and when I put it out, I only ever saw three people playing it.
Looking at Guitar Freaks and Drum Mania, they never caught on either, but Guitar Hero and Rock Band are hot properties. The soundtrack has something to do with that, what with GH's music being straight out of rock history and GF's being a wider range of guitar-centric music. Of course, I'm some sort of freak because I'll take '57 Metallic Grey and Ska Ska No. 1 over Laid To Rest and Raining Blood any day of the week.
That, and the game play was better. One of my friends who spent some time in Japan said, "We have Session over here, and a lot of the songs have karaoke text, so I was just saying we've had that for a while. What makes Rock Band better?" The controls have a lot to do with that. HOPO is a downright godsend, while DM's controller is not too far removed from the nightmare that is Rock Revolution's drum set. Also strangling GF's throat is the fact that Konami's idea for higher difficulty levels means that everything is a chord. (Hey Activision, are you paying attention? I played the GHSmashHits version Woman, and the garbage you added to it was not necessary! Knock it off!)
Getting back to DJ Hero, the *Hero franchise is big enough that its leverage could make DJ Hero more popular than IIDX would ever be. There's also something to be said about the controls. From what I can gather from my friends who DJ, the only thing that a IIDX controler has in common with their rigs is that one of the inputs is a shaped like a turntable.