The Jedi overall, as it says in at least one sourcebook, became TOO complacent, too confident in the stability of the republic, in the senate and its system.
Their arrogance also led them to believe that the Sith were destroyed forever. This is understandable because it had been nearly 1000 years since anyone had heard of a sith. Of course they went into hiding by the new method instituted by Darth Bane 1000 years earlier when, instead of focusing on numbers as the Jedi order did, he instituted the "Rule of 2" where there would only be two sith, a Master and an apprentice.
This way the dark side power, instead of being used up by hundreds of thousands of Sith, would be concentrated in only 2 at any one time, thus, it was believd, increasing their own power. Also, a Master would only pick a student capable of killing the Master. If the student was unable to kill their Master, then that student was simply not worthy of being the next Sith Lord. So a sith would keep searching for a student that they sensed had at least equal or greater potential in the force, recruit that student and train them to eventually be able to kill their Sith Lord Master. If the student could do that, then the Sith and the dark side would grow stronger and be one step closer to eventually being able to take revenge on the Jedi for their defeat 1000 years earlier.
Each Sith Lord understood that they were living and training for a time in the future that the Sith would be strong enough to defeat the Jedi. They would use deception, lies, trickery and cunning, along with politics and the force to eventually take over the Republic and destroy the entire Jedi order. THis, they believed, would usher in a 10,000 year reign of the Sith over the galaxy.
Yoda blamed himself for the fall of the Republic because he was too blind, too complacent and too set in his ways to see what was really happening. He let his guard down in regards to the treachery of the Sith or even if they might still exist somewhere.
Even for a Jedi, once you have a psychological blind spot where you think that something is not possible (in this case because the sith are believed to be extinct), even if you are sensing something off, as Yoda seemed to a couple of times when in Palpatines office, you will dismiss it and it just won't click.
The bottom line is that the Jedi let their guard down as a whole and this is why they didn't see that the Republic was diseased from the inside. Just like a tree that make look healthy and strong on the outside, but inside it's rotting and dying.