i wonder what made them write him off

This has been mentioned MANY times before, and this message board is peppered with Chuck theories, stories, what if's, and what have you.
They just didn't know what to do with his character and always had him eating a sandwich and bouncing a basketball. It was obvious that Fonzie was more like a big brother to Richie then Chuck was since he didn't live at home and wasn't around enough to offer Richie and good advice so they got rid of him.

i think they would have been better off keeping him - mostly as a background character - it would've made for some decent secondary plot lines and he obviously didn't want the big brother/mentor role that the Fonz assumed in the series; I also didn't like the way they dumbed down and over-caricaturized the other characters like Ralph and Potsie. Chuck wasn't dumb, he was just so obsessed with basketball that he ignored most everything else (except food)