Every time a newcomer posts an introduction and somebody tells them they shouldn’t have joined Mastodon.social and need to move to a different server, an angel loses their wings.
Alternative supplementary explanation; certain instance owners don't like that there's such a large instance that doesn't enforce their particular sensibilities and preferences when it comes to what people can talk about.
It's problematic for those admins/mods because mastodon.social has so many users that blocking it is more like punishing yourself, rather than punishing all the people on mastodon.social. It's a power thing as much as it is about spam (which does exist)