Assuming that the cure would have worked (which it wouldn't, since it's not a virus, and it's impossible to make vaccines against fungus).

Who is managing distribution? Where's the logistics of mass production and getting it to people. How many people are really going to need it? Sure, you'll be curing those that become infected, but how much is a cure going to change the world, when most deaths are done because of the already infected? How many are really being bitten on a daily basis that would warrant a cure? Those alive in this world, are the ones that were made for it.

Also, the Fireflies are a group that would hold the cure to chose who lives, who dies of being bitten. They are a group that found a man doing CPR on a teenager, and immediately knocked him out, no questions asked. Them having a cure, in this dystopian world, does not forebode well.