Why do I have to change my username? What's the criteria for deciding who has to change theirs?

So, let me get this straight.. My account, which is about 7 years old, and was created when this game had been out for only roughly 2 years, has a shared username with someone who more than likely made their account LONG after me, but I have to change my username? I am highly upset about this considering that I own my username on nearly every single game which I've ever played, not to mention on many other things, and I NEVER have had issues with duplication or someone else taking it.

What is the criteria for deciding who has to change their username? You're really going to make me change my username on a 7 year old account for someone who probably didn't even own that same username in another region for half that amount of time? Give me break dude.