This particular one would have needed a significant rebuild to continue running into the '90s, or to be converted to a clean energy steam plant. Most of the controls, piping, and valves were of '50s and '60s vintage. Since it was privately owned, and the owners also owned the delivery of gas to most business and residential areas, it was probably a financial decision to de-commission it in the '80s, maybe with pressure from the EPA as well as it was burning all kind of nasties: coal, oil, sawdust, whatever.