Because 3rd parties tend to lump games in terms of sales performance, not genre.
"Wah, wah, Wii owners only buy Nintendo's own games. Pony Land 3 and Ninja Party didn't sell as well as Super Mario Galaxy. Wait, maybe it's because Nintendo gives a damn about quality.... nah, it must be the Wii owners.... hopefully Pony Land 4: Ponies will do better against Wii Sports Resort."
Monster Hunter 3 is the first high profile game that a publisher flat-out chose to develop for the Wii and on top of that, it actually looks like Capcom gives a sh*t because it doesn't like like a 3DO game and they aren't toning down for casual gamers. That's why it matters. When MH3 rapes everything in its path in Japan, 3rd parties should finally wake up and realize that all they have to do is care and anything will sell on the Wii.