My guess is that sales are dropping because everybody already has one. BC isn't needed in the PS3 because everybody has a PS2.
We are too deep into this generation to be bothered about the last, too many good current gen games to be busy with. I have a PS2 and a BC PS3 and haven't played a PS2 game in years. Same with every PS3 owning m8 of mine. Being a complaining minority doesn't mean you're right, sony is wrong and the PS2 is dead.