Xbox 360 and PS3 are pretty much stuck in the sports, FPS and action game genre. That is pretty much it when you talk about the big games, there are a few exceptions like LBP but beyond that the diversity of software is pretty laughable to be honest. Just look at a Wal-mart game case and you'll see a sea of the same, while many may be good games it fails to invoke a feeling of "Wow look at all those different kinds of games!". I may be going out on a limb but I have a feeling that the drop in overall software sales has quite a bit to do with the 360/PS3 lineup. It gets to be like "Oh what game do I get next", let's see, I have Halo 3, Resistance, Gears of War, Frontlines, CoD4, Dead Space, Orange Box, Dark Sector, Bioshock, FEAR, Prey, Unreal Tournament, Battlefield, and on and on. While many of these games are high quality, talk about overload. It gets quite boring and I cannot imagine what someone new to the scene would think seeing all these "mature" looking boxes scattered all around on games that pretty much follow a similar path, which is shoot lots of things in gritty realistic environments (Yes I know Orange Box isn't quite like that but it still is a shooter).
When they look at the Wii they see a game that will maybe help them lose weight in a fun game, an arcade racer with fun characters from the Mario universe, an adventure game starring everybody's favorite elf, a shooter that has more variety than any of the shooters on 360 that doesn't feel the need to create dark gritty environments, a charming trivia game to pass the time at home, a 3D platformer starring everybody's favorite plumber, a stylistic brawler where you have a FREAKEN LIGHTSABER, or how about a zaney off road racer, perhaps better yet a game where you can relax and explore the ocean to your hearts content.
Basically my point is that the 360/PS3 lineup is mainly made up of shooters when you talk about its big name titles (or games that have lots of shooting) while the Wii's big games include everything from platformers, to adventure games, to fitness games, to casual sports games, to brawlers, to shooters, and various others.