I think a part of that is where a person's ability to progess plateaus at. I mean, when first starting out, a person progresses pretty quickly. Eventually they hit a level that takes a lot longer to grow past. It seems that a lot of people hit that level and then get bored, thus moving on. Perhaps part of it is, at least on the arcade side, there hasn't been a new arcade mix in three years (not counting ITG). I know I am very bored with the songs on Extreme, and my arcade took away our ITG machine. I live in a second story apartment, which makes home play quite difficult (not to mention that even with metal pads, it's still not nearly as enjoyable...)
I remember fondly the popularity level from a few years ago, when the game was still reasonably novel to most people. It had its more underground following, but people were more interested and there was a certain energy surrounding the whole thing. Now it seems like a lot of people, especially the ones that helped make DDR as popular as it is, have had their fill of it. Although it's more widely known now, I think the popularity level of a few years ago was the right level. Now DDR is just sort of blending in to the rest of everything and fading away from the arcades.
_________________
Well, the final total was 207, 342. The fuel pump went out, and since it would cost too much to fix, I donated the Buick to charity.
Fare thee well, Riviera, and God speed.