Answer:
Because confederations are by definition decentralized states, and dictatorships need centralized power.
Step-by-step explanation:
Confederations are voluntary unions of federal member states that hold some degree of autonomy. A good example of a confederation is Switzerland, which emerged in the MIddle Ages, and currently has 24 member cantons.
Dictatorships are rare in confederations because dictators need to hold a lot of powers to be effective. A dictator of a confederation would likely attack the autonomy of the federated states, changing the nature of the confederation itself.