GMs are, in theory, the ultimate arbiter of what appears in their campaign, but publishers wield huge influence over what material makes it to the table.
After all, publishers produce and market the products you and I buy. And—of course—powerful commercial forces drive most gaming companies. Speaking candidly as a publisher, it's nice not to go out of business. I want gamers to buy my stuff. I want to eat and buy shiny new things.
But, as a publisher, I also have a responsibility to my customers (and, by extension, their players). Mindlessly churning out books that don't enhance the game or—even worse—break the game in new and exciting ways doesn't help anyone. Overpowered characters, ill-conceived rules or bland and unnecessary options don’t usually add to the fun. In fact, they can suck the fun out of a session and make the game worse.