You’re right that it isn’t a totally new idea, but it is an experimental drug.
ZMapp is only experimental because it’s a cocktail of three monoclonal antibodies that hadn’t yet been tested in humans when this outbreak occurred. Prior to the two American doctors who allowed use because they were in critical condition, the drug had only been tested on a small number of monkeys. It hasn’t even been through clinical trials. This article from the Business Insider confirms that.
ZMapp was released for use because of extreme conditions - of course it took Americans getting sick for anyone to be willing to allow it. That isn’t because the U.S. government was withholding the drug. The two doctors who flew back to the U.S., had to come home to get the treatment at all. The FDA can’t release use of an untested drug in another country. It’s only in the U.S. that the FDA has to grant approval for extreme circumstances. So whether or not people in West Africa will get ZMapp will depend on other countries’ governments.
Fortunately, due to the success of it in these two cases, the company that produces it has stepped up production. Whether or not it gets used in West Africa during this outbreak, they hope to begin human trials later this fall.