For the same reason Wikipedia suffers: People who are really good at writing factual content / good answers are not automatically good at moderation, often they are even some of the worst people for it. Moderation needs empathy, thinking about edge cases, dealing with emotions and so on. Making the people who have the most points on a Q&A site your moderators was always a recipe for disaster.
For a long time, the sheer usefulness of SO overshadowed all of this. People were willing to suffer for the sake of getting a result. But over time the quality couldn't keep up with the pure agony of having to deal with petty dictators. And finally, people just stopped going there, which means the chance that the best answer will be on SO is getting smaller, which means even less people bother with it and so on.
For a long time, the sheer usefulness of SO overshadowed all of this. People were willing to suffer for the sake of getting a result. But over time the quality couldn't keep up with the pure agony of having to deal with petty dictators. And finally, people just stopped going there, which means the chance that the best answer will be on SO is getting smaller, which means even less people bother with it and so on.