Because Hornady 6C brass is supposedly very good, and Norma 6XC brass just increased in price to $.88 per each. Actually, I don't even see brass on David Tubb's website, so now the cheapest is $.99 per each.
Someone on the Hide successfully formed 6XC from 6C with just a FL size, but I'm having troubles getting to the thread from Google. Might have been lost in the upgrade.