Why not appoint Marius dictator?

If Marius needed supreme military authority, why did Rome not appoint him dictator?

The Senate needed Marius but feared giving him open-ended dictatorial power; repeated annual consulships offered continuity with a regular chance to remove him.

A dictator combined the authority normally divided between two consuls and retained it until the assigned emergency ended. The Senate needed Marius's military ability but did not trust him enough to grant that open-ended concentration of power. Electing him senior consul year after year achieved much of the same continuity of command. Annual elections still let political factions end his authority simply by withholding reelection. The arrangement was therefore an emergency compromise between operational necessity and aristocratic fear of a new man becoming kinglike.