Why do politicians delegate?

"Opportunistic politicians maximize the probability of reelection and rents from office holding. Can it be optimal from their point of view to delegate policy choices to independent bureaucracies? The answer is yes: politicians will delegate some policy tasks, though in general not those that would be socially optimal to delegate. In particular, politicians tend not to delegate coalition forming redistributive policies and policies that create large rents or effective campaign contributions. Instead they prefer to delegate risky policies to shift risk (and blame) on bureaucracies"--National Bureau of Economic Research web site.