Because they don't consider people who disagree with their political views legitimate, and wish to wipe them out via any means possible. The fact that the fediverse is small is neither here nor there: indeed, starting by picking off smaller players is a good way to establish a precedent and desensitise employees, so the next round of shutting down bigger players doesn't seem so bad. Boiling the frog, right in front of us.

As for why they do it, this essay may prove enlightening:

https://quillette.com/2020/08/16/the-challenge-of-marxism/