* Most importantly, nobody knows why everyone reanimates and they never will. The zombie plague isn't an important part of the story, it is a plot device to justify an apocalypse in a horror setting. The story is always focused on the characters and how they react to the situation.
In the Walking Dead however on the CDC episode, Jenner says that ..'the French were close,' to a cure.
Maybe they thought they were on to something, but that facility fell as did the CDC, and there's nobody left with that kind of technology. It's also entirely possible that they would've eventually run into a brick wall with their possible solution.
Jenner didn't even know if it was a virus/ microbe/ bacteria etc with all of his resources, he had been in contact with the French and if they knew this basic bit of information one would have expected them share it with him.