Because you're paying more. You could buy more video cards with the extra money thus get more power.
You could buy a gold encrusted 6990 for 10k, sure it gets the same performance but the point is you're wasting money.
Great resale value, though.
I lol'd hard!
@Canaryinthemine
You may have problems with temperatures on those cards (ok not on the 6990...), that might make you turn the oc down -> less MH/s. Also since you want single slot I assume you only have one slot free i.e. these cards will suffocate even more. the only good solution is a WC loop.