Couldnt this be because of the price? I dont know know how the price of old consoles would compare to todays machines but I assume they also started fairly high priced unlike the Wii who started somewhat more mainsteam priced. OTOH GC started at 200? and that didnt sell to well either.
Or maybe its a combination of factors. Personally I think the majority of people who bought a Wii are gamers of wich a part probably jumped in early because of the price or the controller (combined with the price) and a relative small part of ''non gamers'' that buy it.
Atleast I got it a launch because of the price (and because of zelda and the controller, but the price is probably what made me get it right away). I havnt owned a console since the SNES because I couldnt afford a psx/n64 (was about 13) and last gen I was pc only.
Let's look at the launch price and inflation adjusted price of the generations since the Atari 2600 in the USA:
$199 (1977) --- $680 (2006): Atari 2600
$200 (1978) --- $642 (2006): Magnavox Odyssey 2
$299 (1980) --- $800 (2006): Mattel Intellivision
$199 (1982) --- $425 (2006): Milton Bradley Vectrex
$199 (1982) --- $425 (2006): ColecoVision
$330 (1982) --- $705 (2006): Atari 5200
$199 (1985) --- $372 (2006): NES
$140 (1986) --- $253 (2006): Atari 7800
$199 (1986) --- $359 (2006): Sega Master System
$190 (1989) --- $312 (2006): Sega Genesis
$249 (1989) --- $409 (2006): TurboGrafx-16
$649 (1990) --- $1,017 (2006): Neo Geo
$199 (1991) --- $296 (2006): Super NES
$250 (1993) --- $347 (2006): Atari Jaguar
$699 (1994) --- $941 (2006): 3DO
$399 (1995) --- $523 (2006): Sega Saturn
$299 (1995) --- $342 (2006): Sony PlayStation
$199 (1996) --- $254 (2006): Nintendo 64
$199 (1999) --- $239 (2006): Sega Dreamcast
$299 (2000) --- $350 (2006): Sony PlayStation 2
$199 (2001) --- $225 (2006): Nintendo GameCube
$299 (2001) --- $339 (2006): Microsoft Xbox
$399 (2005) --- $413 (2006): Microsoft Xbox 360 (Premium)
$299 (2005) --- $309 (2006): Microsoft Xbox 360 (Core)
$599 (2006) --- $599 (2006): Sony PlayStation 3
$250 (2006) --- $250 (2006): Nintendo Wii
The Super NES, Nintendo 64, Atari 7800, Sega Dreamcast, and Nintendo Gamecube are the only ones that are within $50 of the Wii's launch price, adjusted for inflation. The latter two were in fact cheaper than the Wii currently sells for, but they didn't do so well.
Granted this doesn't take into account price drops over the life of a console, so it's hard to draw conclusions with respect to overall sales. However I figure it's a good starting point.