I think the first Windows compatible iPod was the game changer. It had a large HDD but was much lighter and smaller than similar HDD-mp3 players at the time (such as the ones above).
Part fashion accessory, part music appliance, part computer, the purpose of the iPod was understood by Apple and poorly understood by their competitors.
Just like how the Walkman wasn't the first portable cassette player, or even technically the best, Sony relentlessly innovated on that form factor making smaller, lighter versions, waterproof ones, and ultimately owned that market. It wasn't until the iPod rolled around that someone displaced that position so completely.
I recall it being very difficult to explain to my peers what I was carrying. Thankfully the nomad looked like a CD player.