People join twitter now out of societal pressure. "What is this thing? What is this ad on the subway that says I should follow them on twitter? Who's this athlete that is now Kobe Bryant with a strange @username at the end of his name?"
Developers, techy individuals, and people who consume twitter at a high level are more inclined to pay for it.
People who casually use it to help trend the latest hashtag about #whatmenreallywantfromawoman could care less. They're not going to pay a yearly fee for this, especially when the line between this and twitter is very blurry.
p.s. I am an app.net user and own a copy of netbot.