Because it's all in relation to Jesus Christ.
I know it's not the correct Latin word, but think of it as Before Christ.
You were correct. BC does directly mean "Before Christ".
AD is Latin "anno Domini" meaning "After the year of the Lord".
Bc stands for befofe christ. So it leads up to 'his bday' so descending from our current year. At least i think??
not necessarily the day of his birth, rather the year. it is the catholic calendar, i believe, that we use to this day