Its most likely due to the fact that it was a fresh new keyword that was suddenly launched on the unsuspecting public, and caused HUGE ripples within the gaming world. Drop onto almost any major gaming site and you'll still see debates on whether the name is any good or not.
With the PS3, a lot of domainers probably knew long ago that Sonys next console would, unimaginatively, be called Playstation 3. With the Xbox 360, the 360 tag at the end could be used for a wide variety of things, which limits good domaining opportunities.
With 'Wii' - its a brand new word that Nintendo just invented for themselves, and therefore an open playing field.
I do agree that some of the names being registered now are, frankly, ridiculous, with the word Wii being tagged in front of other single words or combinations of words that have absolutely no relevance to the console at all.
The best domain names were registered within the first few hours of the name being announced. A few diamonds slipped through the net and were registered a couple of days later. Not long after that, some decent typo domains were picked up. It'll be hard to find anything decent that was regged after the first week in May.
I personally did pick up something like 30 of these domains myself, some of which are still in my possession - however I did so within minutes of the press release being sent out by Nintendo, so I feel I picked up some of the better ones (i've already sold wiiforum.com, wiigames.com, nintendowii.net, mywii.com, buywii.com and more)