Promotion is by WGT's decision - no rules fixed.
There are two of them AFAIK - the one who you see is a WGT employee's personal account, thus the promotion may have "internal" reasons. The other one received the tier posthumously, the account being deactivated.
Now you know two ways to get there. Not promising anything, but it looks like you'd have to move, either to California or to Sixfeetunder.
And, BTW, by asking you're the umteenth evidence that their carrot is working fine!