Are Activision really to blame? At the end of the day they are a business, they have one purpose and that is to make money, the company wasn't founded to be a loss making innovator supporting any developer giving them hands full of cash to make whatever they want. They want a return on their investment.
The fact that Activision are the most successful of the publishers goes to prove that despite all the comments here, they have been very successful at what they do, which is to make money from publishing video games.
Their decisions are made because we the consumer continue to buy the software they release, and when a title sells well, they look at the formula for that title and reproduce more the same, standard business sense.