>>714964508It’s because a for a Sith Lord to rule over more than one person, they have to be orders of magnitude more powerful so that they can’t gang up and backstab them. This leads to the problem that the Lord in question has to either deliberately keep their apprentices weak, which makes the next generation vulnerable, or raise a single apprentice who might use their elevated station to gather up their bottom feeders and backstab them, which also leaves them weaker.
In either case, they’ll eventually get their shit stomped by the Jedi who only get stronger when they’ve got an adversary to focus on. Even if another exceptional sith rises up, it’s only a matter of time before one of them is too weak to control the rest.
The rule of two weaponises self interest, since an apprentice won’t backstab their only ticket to power until they’re absolutely certain they’re stronger, and the master won’t get tempted to have a pointless war with the Jedi once they have enough manpower.