Because it's about the most memorable meme from TOS that's not horribly dated, or just plain stupid. An alternate universe of evil opposites might not have started with Star Trek (I'm fairly certain DC's Earth 3 CSA predates it by a few years), but they definitely made a cultural meme out of it. Mirror universe is TOS "thing" for want of a better term. Just like the Picard facepalm is TNG's, and Ben Sisko's motha-fuckin pimp hand (aka the Defiant) is DS9's. So any new Trek content that chases TOS (like Discovery and Enterprise before it) has to do Mirror universe.
A Star Trek about section 31 lead by a mirror universe character... I can't think of anything that'd be more anti-Star Trek.
This series is still in development. Star Trek: Discovery Spinoff About Section 31, Starring Michelle Yeoh, Is Still in Development at Paramount+
They have been claiming that for 5 years now, it was the 2nd series they announced, and btw they keep announcing Star Trek 4 will happen as well and then having the cast of the movie say they have no idea what they are talking about. It sounds like Kurtzman is trying to keep people on contract or get investors again. I'll believe it if it ever starts filming. Till then it's just another announcement to remind people they want to do it.
