Buggy must have something really dangerous that threatens the WG so much.
Come to think of it, I guess Buggy's bounty will be above Luffy. If not, then Law or Kidd should be the Yonko instead of Buggy.
So bounty wise, Buggy > Luffy / Law / Kidd.
Government just knows Luffy is biggest threat among the 3 billion trio, so he is the one who makes it into the Yonko.
Luffy is rank 4 in bounty, while Law and Kidd can't make it. Buggy is at least rank 3.