They can only make a set amount per week or month..... they make more than is necessary during the slow weeks (so while the Wii might only be selling 300k per week now, they are probably making 350-400k.... though the ones they are making now will take a few weeks to get to stores)
Then the extras that are left over are hoarded for the big weeks of the holiday.
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The PS3 is supply contrained because Sony were making a set amount of the new slims, but did not anticipate it to be quite the hit that it is, it will take them a little time to catch up (maybe a few more weeks)
The Wii is supply contrained because Nintendo lowered production levels in 2009 due to the decreasing sales, they had been forced to hoard loads of them.... however the turnaround with Sports Resort/price cut and NSMBWii saw unprecedented Christmas sales, thus they had no stock left after Christmas, it's also possible to air-ship units for emergencies (which means the consoles that were being produced during December were actually sold during december, as opposed to taking a few weeks to ship and sell in January)
Both companies should be catching up soon.
