pop'n music machines cost less and take up less space than a iidx machine. it makes sense to have more pop'ns than iidx's if you're gonna buy multiple machines of something. couple that with the fact that there are a lot of casual players that like to play battle mode together for one credit and no lifebar, compared to iidx where it'll cost two credits and you can fail your first song.
but that isn't to say there aren't places with two iidx's and one pop'n music machine. | 
