Why do Wii exclusives sell more than PS3 & 360 exclusives?

[jonnybmk]

usrevenge said:
1 more hardware sales
2 casual games to all casual audience, no gamer is on wii sorry.
3 Japan
4 good marketing to non-core gamers
5 mario and nintendo as brand recognition. almost everyone even people who are old or never heard of many games know the name nintendo and mario.

Define "gamer" please.