I guess the fun and catty answer would be "because Christopher Cantwell isn't." But that isn't really true. I'm really just throwing that in there because his article is what made me sit up and take notice of Tsu and because hell, any excuse to link to his site is a good excuse to link to his site.
I strongly suspect Cantwell is right about what Tsu is and will become, and that if he isn't right it isn't because he isn't right but because the thing will go under before it has a chance to get where he sees it going. I've seen probably a dozen of these "a social network that pays you!" schemes come and go, and they've all either turned into spam mills or gone under before they could.
But maybe this one will be different. My impression is that they're more seriously funded than most startups of the type ($7 million in venture capital according to Wikipedia).
I use social networks for a number of purposes (making friends and acquaintances, having arguments/debates, finding out about new stuff, promoting my own projects, etc.) and having one more network to post stuff too isn't a huge time sink. It will prove itself worth my while or it won't. If it doesn't, I'll abandon it.
So anyway, I'm going to give it a try. And in the whole spirit of the affiliate marketing thing they're doing here, consider yourself invited to as well.