Ok, I'll bite: Amazon has a massive first-mover advantage in the e-books market. Keeping quiet on their sales numbers is advantageous regardless of how much money they're making. Google pursued this strategy in their pre-IPO days, banking massive (secret) profits from online advertising before the general business culture realized just how much money could be made.

Or Amazon is pursuing a failed strategy, and doesn't want anyone to know about it. I kind of doubt we'd see a third-generation Kindle if that were the case, though.