Each console has their own unique benefit, but I think price is the biggest factor in getting the most console sales
But of course if PS3/360's price were lowered to compete with the Wii, then the Wii's price would get lower too
But we wont see the PS3/360 competing with the price of the Wii ever, because this would mean MS/Sony would lose heaps of money which wouldn't be worth it
Edit: In a hypothetical situation, if you were to take all 3 console, make them all absolutly free, I'm pretty confident the Wii would be last. Obviously they would never be free, but just trying to stretch out my analogy of why I think price is the biggest factor, yet price is something that we won't ever see (atleast enough to have an huge impact on the Wii)