The last century of the Roman Republic had seen constant strife and the civil wars of first Marius vs Sulla, then Caesar vs Pompey, and finally Antony vs Octavian. Quiet Simply nobody wanted to go back to that. Additionally you will find that after the death of Caligula, and before Claudius was acclaimed, the Senate debated reinstating the rule of Consuls, but the discussion quickly became what worthy candidate among them was suited to be the next emperor. Once the position of Emperor was reached, no ambitious man would settle for anything less. With Nero dead there were ambitious men out there with armies who took their troops to assert their right to the position by force and the senate could only confirm them in office, without any army of their own.