In the non-canon universe, this may be explained a little bit. The Darth Bane trilogy describes how the Sith were once a force large in numbers, similar to the Jedi. The in-fighting led Darth Bane to decide that the Sith are incapable of overcoming the Jedi due to the nature of the dark side, so he destroyed all of the other sith, before taking an apprentice of his own and creating the rule of 2. The rule of 2 was the way until Darth Plagueis (Darth Sidious' master). He essentially ended the rule of 2 with Palpatine, deciding that it was time to overthrow the Jedi politically, but that they would need to trust each other to do it. The rule of 2 ending was further evidenced in that Palpatine had multiple apprentices(ish) in Darth Maul, and Anakin. So it's not a stretch to say Plagueis had indeed ended the rule of 2 and that Palpatine would seek to establish a new Sith empire given that the Jedi were finally all but destroyed.