The moment that any group of fans gets big enough to considered a fanbase it turns to shit because more people means more of a chance of weird/crazy people. Small, niche groups are usually fine because there's a smaller chance that a few crazy people will ruin everything, and in the off chance that there some do become vocal enough, they're shamed out of the community due to the lack of support from other crazy people.
Back before the internet was big, this was much less of a problem because there was never really one big community for a show/game/whatever. If something was popular, a large enough fanbase could never be formed. And when the internet did finally become commonly used by the average person, the popularity of these things had already peaked, and the chance for a large fanbase to form was gone.
However, things that appealed to people more inclined towards to technology and early internet adapters did manage to form awful fanbase because of how easy the internet made it for fans to communicate with each other. The best example is Star Trek. Star Trek is the original shitty fandom because of the internet.