Pretty simple, high demand and limited supply at launch, with steady supply satisfying that demand over time we've come closer to equilibrium so that there's actually cards remaining in stock.  In the US at least.  Globally supply is still much lower and prices are still higher due to supply/demand. 

Initial sales and price gouging were undoubtedly driven by reports of limited shipments with no widespread availability until May.  Clearly this has turned out to be false as we've seen constant stock and resupply in the US pretty much everyday at Newegg for EVGA cards at least.

Its important to keep in mind this market segment of $300+ video cards is an extremely small % of discrete graphics card sales, but remain extremely important due to the high margins and profits attached to each sale.  There was never a need for high volume at launch as the number of people who would buy a $500 card to begin with is relatively small, however, I think even conservative estimates underestimated the demand for the GTX 480 which was undoubtedly boosted by its lower than usual $500 price point for Nvidia's highest-end part.