The reason why the term "BC" turned into "AD" is due to the widespread adoption of Christianity throughout Europe and the Western world. As Christianity became more dominant and influential, the use of the term "AD" became increasingly prevalent to mark years in alignment with the birth of Jesus, who is considered the central figure in Christian theology.
The transition from BC to AD was not always consistent and the exact point of change varied in different regions and cultures. However, the usage of both BC and AD gradually became more standardized, particularly after the Gregorian calendar was introduced in 1582, which helped establish a more uniform system of timekeeping.
Today, the designation BC/AD remains commonly used to mark historical periods and events before and after the traditional date of Jesus' birth. While some cultures and regions may employ alternative numbering systems, BC/AD remains extensively recognized as a universal standard for historical referencing in many parts of the world.