Simple really, Eric: they, like me, probably don't believe in paying to play a game you've already paid for. Used vs new aside, I still bought the game, it's mine to play.
1.5M people paid for it? I'm really, really curious how many of those were re-ups from those who bought it new vs used. Or am I way off base here, if you buy it new, do you have it for life for that particular title? Not everyone's gonna by the next years' model, particularly if a) they don't feel it improved enough, b) still play the older one, or c) are not in a financial position to blow $60 for roster updates (a big reality in today's economic environment, I would imagine. Paying $10 for another year of online gaming vs $60 for the new one could just be a reality for some.
Here's another angle: why aren't more developers like Valve? People don't get rid of their L4D or buy them back 'cause of the great DLC (free on PC). They provide continuous value to their games, which is why they can still sell them for $40 new a year later and sell steadily, making their money that way. Maybe that's another business model other developers should adopt, no? Or is asking for regular delivery of quality from the developer too much?
I was there on that fateful day, were you?