No reason. Just a convention these days. Like changing from pounds to grams. Dating regimens are of no concern to me really. But history repeating is
I think you understand it's a hell of a lot more than this. --- Your choice is illuminating.
I remember complaining about BCE/CE over 30 years ago. I was told, "Hey, what's the big deal? It's a matter of choice."
I knew then that today's "choice" would be tomorrow's official "convention" (i.e., the objectively "accurate" way of doing it.)