Lots of Wii's in the wild. Not so many people purchasing Wii software... let alone sports games.
The work involve in creating DLC is not in creating the updated squad. It's in creating and testing the functionality to do this. It was on the list, but there were higher priorities.
I cannot find region sales for PS3 software, but looking at totals for PS3 and the 'other' region for Wii sales, it looks like they could be quite close. I'd think the Europen region would be the largest market for this game. (I'm looking at the last quarter as it would be most relevent.)
When you say DLC is it used interchangeably with a patch because I'd think a squad update would be done via a patch. If so, was it that you decided that the gameplay patch was more important that the squad updates?