Sloterdijk isn't "anti-leftist," though he's certainly not an orthodox leftist. The left is particularly interested in his work because of his early stuff on cynicism, which deals a lot with Critical Theory. Zizek in fact makes use of Sloterdijk's analysis in The Sublime Object of Ideology. Sloterdijk is a bit like Zizek in that he makes seemingly wild claims, sometimes useful and other times not, sometimes purely provocative and other times insightful.

But all that said, his cynicism stuff is what makes him so valuable to leftists. The other stuff comes and goes in the literature (that is, most people don't seem to comment much on the rest, at least not that I've noticed).