- How does he not pick up that Palpatine is a Sith lord?
- How does he not not know Count Dooku is a Sith lord?
- How does he not know a huge army of clones are being manufactured for a period of 20 years?
- How does he not know that the planet of Kamino was wiped off the Jedi’s records, thereby rendering it magically unseen to the law.
- Despite Mace Windu’s warnings, he decide to put Anakin next to Darth Sidious
- He sensed the rage and fear in a young Anakin and did nothing to train/help/mentor him.
- Under his watch, the “one” the “one” who would destroy the Sith — was turned to the Dark Side.
- He had no idea that the “Army of the Republic” had an implanted secret order that would end the Jedi Order in one fell swoop.
- Had no idea that the young-lings were going to be slaughtered
A lot of reasons.
In Episode II, we get this dialogue:
Windu: I believe it is time for us to inform the Senate that our ability to use the Force is diminished.
The exact nature of this diminishing and its cause was left unclear in the films, but in the novelization of Revenge of the Sith we get this:
Though he could not consistently determine the significance of the structures he perceived - the darkening cloud upon the Force that had risen with the rebirth of the Sith made that harder and harder each day - the presence of shatter-points was always clear.
This suggests that Palpatine's power in the Dark Side allows him to cloud the sense of the Jedi. The Jedi are in a fog, unsure of their powers and their wisdom. It's as if they were operating with their eyes half-closed.
Secondly, in the canon novel Tarkin there is mention of another problem: the Jedi Temple on Coruscant was built over an old Sith shrine.
That the Jedi had rasied their Temple over the shrine had for a thousand years been one of the most closely guarded secrets of those Sith Lords who had perpetuated and implemented the revenge strategy of the Jedi Order's founders. Even the most powerful of Dark Side Adepts believed that shrines of the sort existed only on Sith world remove from Coruscant, and even the most powerful of the Jedi believed that the power inherent in the shrine had been neutralised and successfully capped. In truth, that power had seeped upward and outward since its entombment, infiltrating the hallways and rooms above, and weakening the Jedi Order much as the Sith Masters themselves had secretly infiltrated the corridors of political power and toppled the Republic.
The intent was to show the dominance of the Jedi over the Sith, but there was an unexpected side effect — the corruption of the old temple crept into the Jedi Temple, diminishing the power of the Jedi to see what was happening. This happened over a thousand years, never so quickly that anyone could notice it, but it was there.
Thirdly, the Jedi don't know much about the Sith any more. They used to maintain records about the Sith, gather and study Sith artifacts to learn more about their enemies. After the last great conflict with the Sith, however, which took place a thousand years before the films, the Jedi truly believed that the Sith were completely extinct. They feared that keeping records of the Sith could result in Jedi falling once more and resurrecting their enemy. They therefore decided to destroy every bit of information about the Sith that they could get their hands on, trying to ensure the Sith would be forgotten and never return.
Finally, in the Revenge of the Sith novelization, Yoda has an epiphany. The Jedi can't fight the Sith - and it's his fault. The Sith have changed and evolved, while Yoda has held the Jedi still, perfect targets for the Sith.
Finally, he saw the truth.
This truth: that he, the avatar of light, Supreme Master of the Jedi Order, the fiercest, most implacable, most devastatingly powerful foe the darkness had ever known...
He'd never had it. He'd lost before he started.
He had lost before he was born.
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 bu Jedi lore itself, in preparation for exactly this day. The Sith had remade themselves.
They had become new.
While the Jedi -
The Jedi had spent that same millennium training to re-fight the last war.
"Too old I was," Yoda said. "Too rigid. Too arrogant to see that the old way is not the only way. These Jedi, I trained to become the Jedi who had trained me, long centuries ago - but those ancient Jedi, of a different time they were. Changed, has the galaxy. Changed, the Order did not - because let it change, I did not."
That's why Luke was able to overthrow the Sith — he was a new kind of Jedi, not stuck in the old way of doing things.
Yoda was old, while he had experienced much, he had no first hand experience with the Sith. He had also become very set in his ways, which were his masters ways. His biggest flaw was to not recognize and adapt to new threats.
All the Jedi, not just Yoda, are facing a waning ability to use the force. They know something is up, but they don't know what it is or how to counter it.
Sidious was very powerful, and playing a very long game. He undoubtedly out played Yoda, but it was close.
So while Yoda had his flaws, he had managed to keep the Jedi order running for centuries, and to defeat it under his leadership a pretty massive master plan was needed.
Consider that now he has gone it has taken less than thirty years for a new(?) Dark lord to rise I think he did a pretty good job at keeping the dark side at bay.