Always two there are, said master Yoda; A master and an apprentice.

Hence the simplicity of the Sith. While the jedi had a university like structure that integrated itself into the galactic senate, the Sith were organised more into an apprenticeship system. Instead there was a tinkerer who had a mastery of the same elements of the force that were known to the Jedi, and a mastery of talents hitherto unknown to the Jedi. The relationship was always one frought with blackmail.

And why were such talents not used or discovered by the Jedi? Well the jedi follow a rule that the force should only be used for knowledge, events and defence, but never for attack. Force can be used accidently however, although there are penalties for failing to stop once one knows the truth.

A sith was never restrained from using the force for the purpose of attack.