Though Dead Rising was a faithful game to the original, it was awfully boring. I mean the novelty of mowing down endless hordes of enemies wore off quickly. Besides, what's with the glowing red eyes and agressiveness that comes out at night? That doesn't really stick to folklore.
Yes Left 4 Dead changed up the zombie formula, but we have to look at what is "realistic" (where zombies are real) versus what is fun. This follows any game, really. In WWII shooters, sure, you could be in the Russian frontline, pick up a fallen comrade's rifle, and be killed twenty seconds later. However, realism ruins the game. Everyone would rather be a war hero who survived everything from falling back to Stalingrad to the push into Germany.
Same thing with zombies. Eventually, you need a challenge. Dead Rising does this in the way of other humans, and it did it well, but what about people who want a game where the zombies are the challenge and not just cannon fodder? Nazi Zombies answered that, but if you want innovation, you turn to Valve. Sure they were fast and deadly, but they were fast and deadly. It changed from everyone sitting back in their couch watching my determination to kill a zombie with an overstuffed teddy bear (ran out of things to do fast), to everyone on the edge of their seats (split-screen, for the win). L4D was more fun, and the zombies (while unrealistic) were a lot more fun and interesting.