Very interesting explanation that I found online :
The degree is called MBBS, not BMBS, because it stems from its Latin origins: "Medicinae Baccalaureus, Baccalaureus Chirurgiae," which translates to "Bachelor of Medicine, Bachelor of Surgery." Traditionally, Latin was the academic language of scholars and institutions, so many medical degrees retained their Latin abbreviations. This historical convention persists today, which is why we use MBBS instead of BMBS, despite the English full form