A number of reasons; he's a tough son of a bitch, he shows good intensity, good facial expressions and he can sell really, really well when he wants to (although probably wont bother much on a CZW show*).
*when he was jobbing on ROH's HDnet show back in '09, which was my first exposure to him, the first thing I noticed was how awesome his selling was. He made a bunch of guys look really good on those shows, but with the brawling strongstyle he uses in CZW, BJW, wXw, etc, he doesn't really get much opportunity to show it.
I'll never forget that first promo Sami cut on ROH on HDNet. "My name is Sami Callihan, and I'm the new "whore"." (He was supposed to say horror, but came off like whore.)
I like how Mike Mondo, a member of the Spirit Squad, is one of the most over wrestlers in ROH
I like how Dolph Ziggler, a member of the Spirit Squad, is one of the most over wrestlers of the IWC.