I'm looking at charmonium and its decays. Given a list of data on the charmonium states I'm asked to say why ψ'' has a leptonic branching ratio a thousand times small than ψ' and J/ψ. From my understanding this is due to OZI suppression. But I'm having a hard time understanding it intuitively. Why can't any charmonium state annihilate via the strong force to another quark antiquark pair? What is the decay available to the ψ'' that isn't available to ψ' and J/ψ? Thanks!