Because the primary function of the central bank is to control the money supply. If inflation is 0, or more generally if short-term interest rates are always higher than inflation, then the central bank's tools are rather one sided. All they can do is decrease the real money supply by making interest rates higher.

That would be bad news in a situation such as, for example, right now, where a massive credit collapse is greatly increasing the demand for money. To keep the economy stable the Fed must print money. They're doing it by buying long-term debt, which is the best they can do but is disruptive to the debt economy.