I think in a lot of people's cases it was judging a book by it's cover. When you went into stores, you would see the core Nintendo games (which most of them were good) and then a large number of shovel ware titles. So people got the impression that there were only a handful of good games and everything else was garbage. The multiplat games didn't look as good on the Wii, nor did they have the same online functionality as their PS3/360 counterparts (at least in most cases)
Personally the Wii is one of my favorite consoles, but I understand why people had a bad impression of it. Nintendo really pushed the family friendly/casual tone that generation and that pushed off a lot of the gaming community. It had so many hidden gems, but most people didn't know they existed and didn't care to search them out.
