If you're a fan of something and it doesn't sell well. Chances are that you won't be getting more of it.
I'm still waiting on my Mirror's Edge sequal. I bet I wouldn't be waiting if the first had sold a million on day one.
Edit: As for why should people care about it selling more then the competitor. There's probably less of a good reason unless you're worried about the console you support copying some successful but undesirable traits from the competitor (For example, I'd hate the idea of Sony saying "hey getting people to pay to play online seems to work ok for Microsoft").







