Why are CFC’s still harming the ozone layer if they are no longer used?

The ozone layer is a long way up.
The ozone layer is in the Stratosphere. The lowest part of the ozone layer is 20 km up. Any CFCs released take about 10 years to get up there. So once we have stopped using CFCs for about 10 years they will no longer be harming the ozone layer. Since developing countries were supposed to have stopped using it by 2010 that would mean it will take til 2020 before we can expect no CFCs in the Stratosphere.
The other thing to remember is that CFCs were replaced by HCFC (hydrochlorofluorocarbons) which are still sort of CFCs. They do not harm the ozone layer nearly as bad as CFCs but they still harm it. Until 10 years after all HCFCs are replaced by HFCs (hydrofluorocarbons) we will still see some depletion of the ozone layer.