Dual GPUs sell out because they have limited runs and are popular in certain hardcore, niche markets, such as the benching/folding/bitcoin communities. It's not really because the performance is double that of single GPUs. In fact, as has been said, the 680 gives the dual GPUs a run for their money in many benchmarks.
Cards like the 6990 and the 590 are hot, loud, and expensive, and don't really cater to your average early 20s gamer who just wants solid performance in the 2-3 games he plays. Even as a rich, douchebag attorney, I wouldn't drop a grand on a dual GPU card unless I was chasing benches.