When Guitar Hero III was released in 2007, it sold 1.39 million copies across the various platforms in its first week on sale. That generated $US115 million for Activision. When Guitar Hero World Tour launched in 2008, it sold 534,000 copies in its first seven days. That "only" generated $US67 million. That's got to be disappointing for Activision. Sure, you can point to the fact the complete GHWT package is prohibitively expensive in these troubled economic times, but you didn't need to buy the whole pack. Indeed, so many people own a previous Guitar Hero a lot of people could just buy the disc. So what gives? Who knows. Rock Band's probably got a lot to do with it, but I'd never overlook the fact that more casual gamers - and a lot of Guitar Hero owners are just that - don't stomach annual updates as easily as us suckers do.
NPD: Behind The Numbers, October 2008 [Gamasutra]