When a franchise sells, publishers push the developers to milk it as much as possible.
Once people get bored with the franchise and stop buying it, the devs of that franchise will come up with something new.
If a particular franchise sells really well (CoD) other developers and publisher will try to jump on the bandwagon.
Unless people stop buying Crap of Derp 26, that has little to no innovation, apart from slightly shinier graphics, these games will continue to be released again and again.
