FireWire was very widely available on Macs of the era, which is why it made sense amongst other things. (USB 1.1 is very slow, FireWire could also provide power, Apple invented FireWire)
Also worth noting: when the iPod came out, iTunes had been around for a while. Many people had a decent music library on the Mac (from ripping CDs or Napster), and with the iPod, you just plugged it in, and within minutes you had the entire library in your pocket to go, including metadata (playlists, play counts, etc.).
My Mac keyboard had broken the day I got my iPod. I was able to dock my iPod, sync up my huge collection of music in a few minutes and then head out with my new iPod to buy a cheap replacement keyboard.
iTunes had been around for a while if you consider SoundJam MP as part of the iTunes legacy. It memory and a quick Google search serve, iTunes was about a year and a half old when the iPod came out. I guess that's old for CD-distribution days.
The best part about FireWire (from the perspective of someone who never actually used it) was that it was very tolerant of uneven voltages; thus, all iPods that could use it (which went at least through the first Nanos) had very, very inexpensive car power adapters - they could just use the "12V" that's really more like 13.4 V. No real circuitry inside, just a cord to plug into the iPod.
Yeah but Macs were pretty close to the bottom of their popularity in 2001 and Firewire was near non-existant on PC. It was really only a high-end PC thing or maybe an add-on for those FW capable camcorders. When the iPod launched USB 2.0 was already out, available on okayish PCs (not Macs yet though) and was pretty much fast enough as was seen with later USB 2.0 capable iPods.
