The longer-lasting zombie franchises tend to do so IMO because they don`t delve into the WHY of the undead, just that they ARE, and dealing with the human drama that comes as a result of the apocalypse. I always figured that Romero would have faded into obscurity long ago if his second or third movie were about a team of scientists trying to find a cure, or at least figuring out how the things move and moan. I mean, we don`t really KNOW how the warp drive or transporters work, right?
Here, we had the CDC but that mostly served to identify the cause as a virus (and ultimately to confirm that everyone HAS it), but that`s it. Our heroes don`t have the means to do anything more beyond survive and that`s what we hope we`ll be enjoying hopefully for at least a few more seasons* .