10 Crazy In-Universe Rules Star Wars Characters Must Follow
4. Jedi Weren't Allowed To Learn About The Sith
The prequels gave viewers a certain insight into the working politics of the Jedi Order. It was essentially a dogmatic cult with those at the top having the final say in all matters, and any who refused to get in line being ostracised. Even a respected Jedi like Qui-Gon Jinn was denied a position on the Council due to his unorthodox way of doing things.
By the time of the Clone Wars the Jedi had lost their way. Peace had reigned for so long that they became complacent, arrogant, and dogmatic. One of the ways this manifested was by preventing Jedi from entering certain sections in the Jedi library, specifically restricting access to Sith artefacts and knowledge.
The Jedi Council believed the Dark Side was so corrupting that anything associated with it was too dangerous to tamper with, even for academic purposes. This ultimately led to the Jedi Order becoming so out of touch with certain aspects of the Force, that they failed to sense the presence of the Dark Side and the reemergence of the Sith.
It's ironic that for a group who considered fear to be the most corrupting emotion, it was the fear of knowledge that ultimately led to their downfall.