The Jedi were deceived by the Sith lord Darth Sidious for years and years. Sidious, using his double identity as Palpatine held various political offices for years until eventually grabbing the highest: Emperor.
With the fall of the Republic, the formation of the galactic Empire and extermination of the Jedi, Sidious completed what he had set out to do many years ago.
How could the Jedi become so blinded by a Sith who was right there in front of them? Literally in the same room with Jedi many times throughout the years! Sidious was well trained from his own Master, Darth Plagueis, in disguising, hiding his Force abilities and Force signature to the point that Jedi could not pinpoint the Dark Side of the Force that definitely should have been radiating off of him.
Darth Sidious was very strong in the Force and apparently in deception and cloaking abilities as well. Sidious was always skilled enough to evade detection by throwing pursuers off his track, until it was no more and inevitable.
Palpatine versus the Jedi and the Republic
The Sith Lord was literally so close, puppeteering the events and playing both sides of the Clone Wars until it was all over. Eventually, the damage was done, just as the Sith had planned.
The Jedi and the Republic were blinded by the Dark. By the time anything could have been done to remedy the situation, it was far too late and the situation far too grave. The Jedi had played right into the evil plans of the Sith and paid the price. The Dark Side overcame the Light. Darkness and evil extinguished the light that was the Jedi for many years.
That was until the hope born in the form of Luke Skywalker emerged to challenge the Sith and destroy Sidious with the unforeseen help of his father, Anakin Skywalker. For the Sith had underestimated the very thing that they had sought to destroy from the beginning: love. Love even in the darkest of times shone a light in the dark.
But then, as we know, Sidious returned. And Rey, with a little help from Ben Solo, taught him this lesson all over again.
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