We need to first look at both the Jedi, and Sith philosophies, as well as their training.
Firstly, per both Disney Canon/Legends, the Jedi(expect maybe in the Old Republic) primarily peacekeepers. They did not desire to be the best of the best, aside from perhaps friendly peer competition between padawans, knights, etc. They are also not in favor of focusing power, and aggression in combat. This is why forms like Juyo are considered taboo among the Jedi Order, since they focus on pure hatred, and aggression. Per Star Wars: Tarkin, it states that there was a hidden shrine that clouded the Jedi's vision.
If you look at the Sith, there are reasons why people like Darth Malgus, Maul, Sidious, Count Dooku, Vader, and Vitiate each have earned their own right as powerful force wielders/dark-side warriors, or both. The Sith Order focus on pure and aggression. Even if we look at the Sith Code(EU Legends), it clearly evidences the fact that the Sith care about power, and are the direct polar opposite of the Jedi Order. Now, we need to look at the Order of Sith's history.
The Order of Sith Lords' purpose was to infiltrate the Republic, and lead to their destruction. They had centuries of being able to build a power base. Sidious' master, Darth Plageuis(in the EU) was pretty powerful, and influential in the galaxy. He was able to teach Sidious many things, and since Sith training focuses on hate, and power, a Sith would naturally become more powerful as their expand their knowledge/ grasp of the dark-side of the force.
Even in Disney canon, Sidious was quite influential. Per Star Wars: Attack of The Clones, Dooku stated that Sidious controlled many senators. He was also a chancellor of the Republic, as well as arguably the leader of the Confederacy of Independent Systems. Being the leader of the two largest factions in the galaxy makes you more or less have a large amount of resources to be able to gain more knowledge. Being the leader of this two factions didn't affect him at all, clearly exemplified by the fact that Sidiouis was able to sneak in and out to play his alter-egos perfectly.
To put it simply, I'm pretty much stating that due to the points I stated above, Sidious pretty much had a strong foundation for him to be pretty strong. Why other Order of Sith Lords members couldn't match his power in this manner, Idk the answer to.