This point is mirrored in EyeToy. The original Play shifted millions of peripherals. Subsequent Play's were better games, adding coop for one thing, but were reviewed and bought far less because the novelty was gone. "More of the same" doesn't sell units over the long term. The games need to evolve.
I often wonder about this because real games don't change much.
Tennis hasn't changed, football doesn't change, chess hasn't changed nor has backgammon, golf is played now as it has been for over a century....
They all have very simple rules, the appeal lies in accessibility and the potential to improve your skills.
Why has computer gaming to date been so novelty driven? Economic reasons are obvious - produce a new shooter, reap new dividends from sales. But do gamers really have the same desire for novelty? Some do, obviously. And the industry will tend to accumulate these, as the people who prefer to build long lasting skills will gravitate to other pursuits. But we see in the success of mutually different games such as Starcraft, Sims, and to some extent WoW, that novelty isn't really necessary to be wildly successful, it suffices that the values you offer are compelling enough.
My feeling is that the industry generally cuts its own potential short by being so narrowly focussed on selling the old concepts and franchises in new and ever more expensive clothing.