This is due to the fact that interest rate measures work through to inflation via all sorts of complicated macro-economic processes, with inflation right at the end of the chain. Direct inflation targeting, too, calls for a sufficient measure of stability of these relationships. Obviously effectiveness is in any case highly dependent on the quality of the inflation forecasts.
Moreover, the central bank must be free to react pre-emptively and adequately to any deviations between the desired and the predicted rates of inflation. In the second place, it must be possible to hold the central bank accountable for the consequences of its policy. It must account for its decisions and explain them to the public.