It's really a matter of scale. If you own a group of stations in a larger market, then costs are spread among the stations. That includes utility costs. If you're a small market, stand alone, full Class C FM earning $16K a month, transmitting via a 30kW tube transmitter, that $3,000.00 a month power bill really cuts into any profits.
AM tube transmitters are incredibly inefficient. A 50kW AM station still using a tube transmitter will cost $8K-13K per month in utility power, depending on per kwH utility rates. Solid state AM transmitters are much more efficient, but could be cost prohibitive for a smaller station to purchase while just trying to make ends meet.