I'm not sure the diversity matter so much as the scale. Once you've got a very large audience, it becomes possible to scale abuse on it.

When each community is small, you have to put in a lot more work to say, spread disinformation so there's natural censorship on it.

Put another way: humans evolved living in fairly small groups that were often related to each other. Their brains are optimized to operate in that environment. Put those brains into an environment where the number of other humans is orders of magnitude larger and most of those other humans are not physically present and not related to you and...that brain doesn't work well.