Answering the OP's question from my point of view...
I think iPods have become more popular at Head-Fi mainly because of three things:
- First of all, the iPod itself has brought a lot of people to Head-Fi. People who had just bought an expensive portable player, and wanted better headphones to go with it. I for one found the site this way.
- Secondly, people have found ways to make the iPod sound better. The line-out (first the Pocket Dock and Sik Din, now mostly Turbo Dock), the portable amp boom, the iMods (hardware iMod by Red Wine, various software mods), better compression codecs... All of these things have made it easier to achieve good quality sound using the iPod as source.
- Then of course the iPods are getting better with each generation. Better batteries, sleeker looks, new abilities, more memory.
Some people really seem to dislike the UI and design of the iPod, but I guess the majority still thinks that iPod's iconic looks and the really simple and intuitive UI are definitive pluses when choosing their MP3-player.
And last but not least: The iPod is already a classic. It used to be FOTM, but now it's a standard. In ten more years it'll be retro cool.