The answer is gamers as a whole. If difficult games were flying off of the shelves, then developers would start making more of them.

Defs Nintendo and FB

One of my favourite series that got dumbed down -> Rainbow Six series. The original ones where you could get killed in 1 shot.

Playing MULTIPLAYER on Rainbow Six was a blast.

People like this guy... he bought Wiifit and Just Dance...

Everybody who bought the games in the OP instead of God Hand, Vanquish or Bayonetta, for example.

EDIT - Buying Mario or Just Dance have very little to do with it. People who buy casualized games that belong to the same genre do.

I think you all have a rosey colored view of game history here. Until the PS1/2 generations, games have ALWAYS been casual. The entire video arcade industry was built upon casual, easy to play coin op games. Dig Dug, Galaga, Centipede, even Super Mario Brothers; these are the kind of games you see on your iPad or android now. These are also the games the industry was BUILT on. They were a challenge to play, but also easy to pick up and play. They were designed to eat quarters, so by default they had to be as accessible to the widest variety of people as possible.

Then the pendulum began to swing into the "hardcore". SNES and Genesis games were more complicated, but still easy to pick up, for the most part. Playstation and N64 ushered in the 3rd dimension, and players from the 80s didn't know what to do with it. It wasn't any longer a choice between going left or right. There was no longer a clear objective to the casual player. Mario's objective was start level, go right, and try to get to the end of the level. What was the Objective of Parasite Eve? Or Final Fantasy VII? We had more kids (at the time) playing games, but they had lost their parents as a playmate. My Mom loved to play pacman, but doesn't have a clue when it comes to any new games. Arcades died at this time, because the "popular" games weren't arcade friendly. How do you make a coin op out of games like Tomb Raider or Quake? If they aren't arcade friendly, then they aren't casual friendly either. When the fighting genre fell out of favor, the last "pick up and play" genre, arcades finally died. The hardcore had beaten the casual business literally to death.

But its not because those people didn't want to play games, its because they didn't want to play the games being made anymore. As dev costs for games grew, the market for those games was also shrinking, because every year someone else who was a "hardcore" gamer started a job, or got married, or had children, and the time they had available to dedicate to a "hardcore" game shrunk.

So the pendulum began to swing back the other way. Small and indie dev studios with super small budgets now have a publishing avenue thanks to the Apple app store, android market, and now on consoles with the eShop. They are making the games that people want to play, that casuals want to play, that arcades used to have. Angry Birds, Fruit Ninja, and many more games aren't the result of "dumbing down" games, but of returning them to their roots. Will the pendulum stop swinging and land in a happy balance in the middle? Who knows? In another 5 years the pendulum may swing back toward the "hardcore".

Who is to blame for casualization? I think the question is inherently flawed. The real question is, who's responsible for the hardcorilization of a huge and thriving market, to its detriment, and who should we thank for bringing the industry back to its senses?

^Nintendo = who we should thank. =)

Left? I've found an evil clone from the mirror world! Quickly, catch him!

Stupid scumbag brain.  Fixed.

Currently it is obviously Wii Sports(and its ilk) and smartphone games. Wii throws out this big net to catch all kinds of gamers, makes a shitton of money. Other profit seeking companies see the exodus of new gamers and wonder "how can we get them to buy into our franchises?" Reason they are not playing Resident Evil(example) its too hard and complicated. How do we get the sales of a game with an established name more fans, fast(cash grab!): easy up the game so that the casuals can get it on it. "What about our established fanbase?": Well they are mindless idiots that will buy any game from this franchise. "So we will get the old idiots and new idiots and make record profits?" Yes!