Not sure why this is a mystery for some. It's already explained why Sith Lords are stronger than the Jedi during the era of the Prequel Trilogy and after; ''Darth Bane had gained more knowledge and mastery of Sith techniques and power than anyone who had come before him. He used that knowledge and skill to change the Sith Order forever.'' ----Source: The Official Star Wars Fact File Remake #022 (2014) ''The Sith had changed. The Sith had grown, had adapted, had invested a thousand years' intensive study into every aspect of not only the Force but Jedi lore itself, in preparation for exactly this day. The Sith had remade themselves. They had become new.'' ----Source: Star Wars Episode III - Revenge of the Sith Novel (2005) "As they gained knowledge of the Dark Side of the Force, their powers increased with each generation." ----Source: The Phantom Menace Scrapbook Instead of seeing countless Sith, and multiple different Sith organizations attacking the Jedi order and other planets, and seeing countless battles between the Sith and the Jedi, the Sith decided that they can't win against the Jedi and others due to infighting. Because Sith lords also would fight with each other to get more power, while the Jedi don't fight with each other, so that naturally gives advantage to the Jedi. Sith made a new plan with Darth Bane, there could be only 2 Sith lords. And with this, with every generation, the Sith lords has grown more powerful. And with Sidious and his apprentices, (with the expection of perhaps Plagueis) we've seen the most powerful Sith lords ever against the Jedi. It's not that Yoda, other Jedi are weak, it's the Sith has grown more powerful than they were before. Could they beat them with different circumstances? Like cheap-shotting etc? Kenobi did it with a cheap-shot against Maul, when Maul wasn't off-guard he took both Qui-Gon and Kenobi at the same time and beat them, killed Qui-Gon and disarmed Kenobi, they had no chance of winning in a direct battle, or Windu used an amplification to defeat Sidious, Windu wasn't able to save any of his Jedi friends, all 3 died, only later he become more powerful during the fight due to other factors, but they couldn't do it again under normal circumstances. Which is why they don't win against Sith in a direct battle, when everyone is at their usual power level.