Was a reason given? Because in my head the perfect reasoning to go with amongst the Sith would be someone at some point saying "You know what, after all that time and all that planning that started with Darth Bane, while eventually Darth Sidious succeeded in the Sith's revenge and taking over the galaxy, it only lasted two decades. So screw it".

Quote Originally Posted by Carabas View Post
We witnessed the disbanding of the Rule Of Two at the end of Return Of the Jedi, when the Sith went extinct.
Well of course technically when something doesn't exist anymore there are no rules whatsoever to enact. I'm operating on the possibility that someone(s) somewhere down the line, will re-establish the Sith. Just like eventually the Jedi will be re-established.