Wired has an article up about the Mac Mini, and why Apple has neglected serious updates to it for so long. I don't know the answer to that one, but I think I can take a stab at this, which comes at the end, asking "what do people buy it for?"
And though that small number is unsubstantiated, it would make sense given the responses ZDNet is receiving from Mac Mini owners regarding what they do with their puny desktops. The examples include cheap server setups, digital music servers for audiophiles and replacements for Windows PCs. But, as ZDNet also notes, those are all niche markets.
I suspect a lot of people get the mini for the same reason I originally did - it's less exppensive, and you can re-purpose existing hardware (monitor, keyboard, etc) for it. It's a toe dip into the Mac waters, without having to make the commitment required by an iMac or a Macbook. I eventually bought two more Macs (and I use one at work as well) - all based on that initial mini purchase. If I had to guess, I'd guess that a lot of the mini buyers represent the leading edge of new customer acquisition for Apple (in the computer space - in the mp3 space, they are already way, way ahead).