There is a market for both types of products. Nintendo is doing their own thing while MS and Sony are doing theirs.
It's wrong to conclude that MS and Sony are donig something wrong because Nintendo is doing something right.
The market is based off price levels. Nintendo is going after the mass consumer without any competition due to their price level where as Sony and MS are fighting amongst themselves for a niche market until their prices drop to a certain level.
Think of it like this (premium consoles only):
@599 you can cater to 5 million potential buyers
@499 = 10 million
@399 = 20
@299 = 40
@199 = 80
Using that baseline, MS and Sony can keep fighting but until they'll never hit that extra 40+million sitting around at their current price range. Nintendo is not only taking the enthusiast gamer who get a Wii along with a 360/PS3 but also going right at that range that MS and Sony have priced themselves out of. Ofcourse, both sides will eventually drop their price but by that time, Nintendo will have had saturated the market and ensured that their console has sold regardless of what the other two are doing. Let's not forget, the Wii is not a subsizied unit so they just made a profit on about 3.4+ million units and counting? How long before MS and Sony can make that claim?
One could even argue that 30million Wii's sold, along with the games/accessories attach rates for each console, could be more profitable than 50million 360's or 60million PS3's over the course of their lifespan.