Probably because they stopped making them... The 6xxx cards have been disappearing, i doubt alot of people are still making them since the 7xxxs are out. Wait for a 7850 instead, better power usage, less heat, and you'll be able to find anotherone if you want to crossfire later.
When are they going to release them? Im starting a new build right now, and i need the best GPU i can possibly get on a budget.