Although it is generally not the case, distributive policies can, however, be delegated to unelected bureaucrats if they can be instructed to be fair behind the veil of ignorance. The second reason is efficiency. Central banks' limited number of instruments should be directed at a limited number of objectives, with a clear priority.
For an assessment of why distributive policies are usually not delegated to bureaucrats, see A. Alesina and G. Tabellini, 2007. “Bureaucrats or Politicians? Part I: A Single Policy Task”, American Economic Review, March, p. 169–179. BIS central bankers’ speeches  1  have significantly impaired the potency of our standard monetary policy actions.