There are two simple answers.
1. non gamers buying for wii sports and mainly wiifit. Heck my parents hate and despise video games and they bought a wii and wiifit on ebay for 100 dollars more like a couple months ago.
2. for gamers - if you don't have a wii and look at the library there are plenty of great games to buy for it. sure for people that like to buy a game like every couple weeks and are always complaining about the wii has no games and is collecting dust. Well who gives a shit, you already have a wii. It is worth the 250 dollars. It has at least 10 great games for it.
So there ya have it, gamers who don't have it see a big enough library to justify 250 dollars and grownups who don't like video games are buying it for wiifit, ect.