Nintendo has never released their main franchises frequently. If you bought Wii expecting more than 1 Mario game, 1 Mario Kart, 1 Smash Bros, 1 Zelda after TP, etc, you're a fool.
Even with their big breakthroughs of the last few years, they aren't milking them with frequent releases. Nintendogs has no sequel announced. Brain Training had More Brain Training, but nothing beyond that. Wii Fit has no sequel announced. After Wii Sports and Wii Play, it takes 2 1/2 years for Wii Sports Resort. Animal Crossing will have just one version each for DS and Wii. Even Mario Party has backed off from being a yearly franchise.
This is just what Nintendo does. Even if it relies on old technology (lateral thinking of withered technology), it takes longer to invent "the next big thing" than to pump out a sequel with an existing engine and assets. The Mario team will probably take 3-4 years to make the next Mario game because they're hunting for an idea as cool as the "gravity" concept. Miyamoto has talked about how right now the Zelda team is looking for things to shake up the series, and before that suggested that Twilight Princess would be the last Zelda of its type. And even a game which could have half a dozen sequels by now, Wii Sports, will only get a sequel for Motion Plus.
Companies like EA, Activision and Ubisoft rely on frequent sequels. The Sims 2 has had 2 expansion packs per year. The Tom Clancy franchise has at least 2 games per year. Need for Speed, Medal of Honor, Call of Duty, etc., come out yearly. Guitar Hero and Rock Band are going for 2+ and 1+ games per year, respectively. For Wii, Raving Rabbids and MySims are becoming annual franchises.
"[Our former customers] are unable to find software which they WANT to play."
"The way to solve this problem lies in how to communicate what kind of games [they CAN play]."
Satoru Iwata, Nintendo President. Only slightly paraphrased.
