It's expensive to license, like ol' Love Slave mentioned.
Plus, this database is so huge, due to the amount of posts and members, that it would take literally half a day to convert.
Like has been mentioned before... we've considered upgrading, but I'm not gonna spend the 12 hours required to do it, and I'm certainly not going to do it if it's something that's got to be paid for first.
This board is fine. If it ain't broke, don't try and fix it, yeah?