I find this a little bit interesting, having been one of those people myself. There are essentially two reasons behind this. 1) preference for a format not supported on the iPod and 2) Apple resentment. I'll set aside 3) DRM because if you have DRM'd music that creates a naturally high barrier to departure and there's two kinds of DRM, each falling into the other two categories ("I can't play may PlaysForSure/Zune music on an iPod "or I hate FairPlay, but willingly subject myself to PFS/Z").
I was one of the former, preferring Ogg Vorbis over MP3. If not for DRM'd PFS/Z files I don't really know why anyone would actually prefer PFS/Z. Some people prefer Ogg because it's "free". While it's true that there are no licensing fees like there are with MP3 it's not entirely clear wether Ogg is free from patents owned by Fraunhofer. These people are also known as Freetards. News flash, consumers don't pay MP3 licensing fees and whichever "alternative" music player you bought that supports Ogg also paid the MP3 licensing fee. Get over it. Then there are people who prize MP3 and loathe AAC. These people also fall into the category of Apple resenters, think that this view point is cool. These people are also known as delusional. The fact is that AAC is lesser known as M4A which is the successor to MP3. In other words the only reason they're not called MP4 is because MPEG wanted to distinguish video (M4V) vs. audio (M4A). AAC is the name of the codec standardized as M4A. It's not Apple's codec. They just use it as do many others. Some people mistakenly believe that MP3 is the "baseline" or "standard" and is compatible with all players. This isn't true. I don't know of a single device on the market that doesn't support M4A (including all PFS devices and the Zune).
Now, I've known no less than 10 of these people and, as I mentioned, used to be one myself. I had a Rio Karma. I was a freetard. My main reason for getting the Karma was Ogg support. but the thing was a brick. It was more than twice as thick as an iPod and much heavier. I also dropped it onto a carpeted surface and the little wheal that stuck out snapped off. This made the already poor UI impossible to use. The repair price for that supid little wheel was more than the price of an iPod which didn't have vitally important parts both sticking out and extremely fragile. In fact the iPod was plagued with neither of those problems. I would also frequently have conversations with people that went like this:
Them: What's that?
Me: My Rio Karma
Them: (blink, blink) What's that?
Me: (sighs) It's like an iPod.
You might as well try to explain to someone why it's not a Frisbee.
Whenever I meet these AntiPod people I say something to the effect of "Oh, you have a (not an iPod), hows that working out for you?" and they generally replay something to the effect of "Well, has some features but it doesn't have others, I'm probably going to get an iPod soon." Generally within one month they have a shiny new iPod and are very excited and/or happy about this turn of events. I currently do not know a single music player owner who is not an iPod owner.