Hey how does this sound.
Each console has key niches in the market where they enjoy great strengths where compared to other consoles. In these niches they enjoy great success.
Nintendo has several large niches where the competition just isnt considered viable, they have the simple gamplay type games down to an art, the lifestyle type games such as guitar hero, wii sports etc they have an advantage and they have a great price and a keen following from people who've owned nes-gamecube. These are all niches where the competition LOOKS like it doesn't exist, because its irrelevant.
The PS3 and the Xbox360 compete for a lot of the same niches, the Xbox360 has better offerings for people who want multiplayer, people who don't like motion controls obviously won't buy a Wii. The PS3 has the Blu ray whilst the 360 is better on games currently.
Now there is some overlap but for a lot of Wii purchasers the Wii is the only console to consider. Look at how the Xbox360 dropped the price below the Wii and Wiis sales weren't even changed. People obviously value the Wii more highly than the Xbox360 or the PS3 because even at the same price they still buy the Wii.
If the Wii is $250 but you would pay $300 for it and the PS3 is $400 but you'd only pay $400 for it and the Xbox360 is $250 but you would only pay $250 for it because of how you value the console, then obviously the Wii is the best value for you.
Not completely holeproof but its my take on it. Think economic value - How much you'd pay for a console. Remember those people willing to pay $800 for a Wii? Thats how much they value it!
Edit: I hate saying this, because personally I would love to never see the Wii in the top 50 so I can easier compare the Xbox360 with the PS3! I get bored honestly with seeing Wiisports week after week! Its because the conclusion has been made! I want to see the information where the competition is still up in the air!