Well, looking at the worldwide charts, I can see that Wii software sales are dominated by the same handful of games week after week.
Seriously, what else do they need to display other than Mario Kart, Wii Fit, Wii Play, SSBB, Galaxy and a few new releases (Wii Music, Animal Crossing)? Just a theory :P
Can't say it's the same story in Australia. Wii and DS games take up the majority of shelf space in my area.