It was successful immediately because there literally wasn’t any other player in the world that had its capacity and physical size.

Everything else lacked mass market appeal because it couldn’t hold enough songs or couldn’t fit in your pocket.

Not to mention the vast majority of the population didn’t know how to pirate music, and most music stores were shit compare to iTunes(and that is not a great endorsement).

The only huge barrier to adoption was the initial FireWire only model, but I’d be willing to bet even with that restriction they sold more units in the first year than any other model of music player.