My guess is that it's the old laws of supply and demand-- by the time Burning Crusade dropped, everyone knew it would be a hit, so Blizzard made a bunch, while the original CE probably had a lot fewer copies made. But the noncombat pets might be a draw as well-- the original CE had Diablo, Zerg, and Panda pets, while the Burning Crusade only had a pretty unimpressive Netherwhelp pet.
And the other interesting question is what will happen when the next expansion drops. Being that WotLK is much more integral to the Warcraft that everyone knows and loves (while BC was much more involved with the ancient history of Warcraft), will people who've skipped Burning Crusade so far be more inclined to pick it up when Wrath releases? Maybe the BC:CE will get rarer as we get closer to Northrend.
[ via WoW Ladies ]