I think perhaps IEMs are becoming more popular because the quality and sound ability is increasing, and more people who listen to music while commuting, be it a bus, train, or carpool, are starting to take advantage of them whereas before, they'd either have stuck with normal headphones (I would imagine supra aural being the most common) or the stock earbuds, or have not listened to music at all.
Also, I think portable music players are becoming much, much more widespread (to the point where we see phones marketed with their music storage capacity as a major point), and that is, in turn, driving up IEM sales.
I'd personally be interested in seeing a sales analysis between historical and recent headphone sales, and historical and recent IEM sales, to see if "normal" headphone sales have actually diminished, rather than staying the same while IEM purchases increased quite a bit.