Mbin is not as popular as Lemmy or Piefed, so nobody talks about it. Even before Piefed started being popular, Kbin was still not that popular compared to Lemmy in the past nor Piefed right now (might be an exaggeration).
With the Kbin lead dev, Ernest, not being active enough (due to medical reasons), the community forked Kbin, which is what we know today as Mbin.
https://codeberg.org/Kbin/kbin-core/issues/1383#issuecomment-1999046