That was only recently, like in the past few months. It takes a long time to synthesize the stuff, as right now they're manually injecting individual tobacco leaves to get the right antibodies made.
You need a vaccine...a prevention device not a "possible cure" after spreading the damn thing all over the place.
I don't think a vaccine is crucial in the short term since it's not an airborne virus. Clamp down on travel, fast track manufacture of ZMapp and treat those already sick. The drug ostensibly works; those that have been treated with it lived, even far down the path of infection. The guy in Dallas who didn't get it died.
A vaccine would be nice, but that's probably a ways off.
They have no idea if it was the Zmapp that “cured” them. They also received transfusions from people who had survived.