So, why hasn't the world adopted this calendar yet? It's not a hard transition, just add 10000 to the current year.
What's the advantage, you may ask? Well, it provides more historical context. A lot happened before Christ. Ancient Egypt, ancient Greece, ancient Sumeria, ancient Babylon, ancient India, ancient China, ancient Mesoamerica. It's kinda like all of this is just a footnote when it has the letters "BC" appended to it.
"A common mistake that people make when trying to design something completely foolproof is to underestimate the ingenuity of complete fools."