He doesn't say so, but I strongly suspect this has at least a little something to do with Sinclair broadcasting's purchase of the station. When they take over, if past purchases are any indication of future actions, they will force all WGN broadcasts to show right wing propoganda as part of their newscasts.
I'll miss Suppelsa on TV. He's an actual journalist, but not one that took himself too seriously. I interviewed him previously for Shore Magazine. You can read that interview here.
