We do see other fallen Jedi though and get a sense of why they fell. Duku is pretty open about his feelings on what he felt went wrong outside of wanting more power. People tend to push that aside and just react in horror that he was under Palpatine for a time but his reasoning was sound to a logical person. The Jedi were corrupt. They were hypocrites. Ectect.

Perhaps it is because we are conditioned by the canon we are given to think all fall in the way of Anakin and Barriss ect in that they will fall quickly. When perhaps the reality is that more break away slow, jaded and the Jedi see them as walking away and not so much as a threat. The fewer sith there are the fewer there are to tempt Jedi but in times gone by...in the hayday well then it would have been war with people switching side all the time and the need for later jails I'd expect.