They are curating their instance in a particular way which is the very thing that the fediverse and lemmy allows for. Some people are butthurt that they aren't included.

The slightly more detailed explanation is that there has been a big increase in bots and trolls and due to a lack of moderation tools they opted for defederating with instances that conflicted with their goal. That goal seems to be an easy going and tolerant community. By nature this goal means that it will be a smaller community and for some reason that has angered people.