I think this depends on where/what platforms said fandoms are on. If you’re on somewhere with a lot of performative liberals, “antiship” largely ““won”” those spaces and, combined with aforementioned performatism (ie lots of virtue-signalling), naturally leads to a lot of reactionary tendencies.

edit: I should probably define stuff better. “ship” means character relationship pairings, many are “canon” as in appears in the source media, but many ships are not canon, or have altered scenarios, such as age or race or what have you. “proship” basically means, “don’t like, don’t read” - just leave it alone. “antiship” is its antithesis, and takes on a keyboard “social justice” warrior mantle, and demonizes ““problematic”” ships, frequently “antishippers” go around and harass fandom creators who create content they find distasteful/evil – the most common scenario is claiming x or y thing in a fandom with fictional characters constitutes abuse, and then assuming it means the person who created depiction of said thing “supports” or does said thing IRL.

The “performative liberal” aspect - typically consists of over-emphasis of identity politics and oppression olympics, often to where their id intersections that have privilege are downplayed (whiteness, masc, cis, etc) and their underprivileged ids are forefront.

Additionally, I’ve noticed that fandom spaces filled with the above tend to treat Fandom as like, the Hegelian center of the world from which ideas and ideals spring forward and materialize into the real world, where the power of fiction and ideas are what shape the world we live in, rather than fiction largely being reflective of the authors inherent biases and conditional on their own relationships to structures IRL.

Combined factors above, you get people who think policing content - content type, access, availability - is their greatest duty, particularly as it signals to others about how righteous they are.