Perhaps they cant afford the extra processing equipment to do stereo.Im sure a stereo encoder is big bucks.
On an odd note ,a analog station here in NYC \ does MONO on purpose! WEMP 101.9,I know for a fact they Do have a stereo encoder and they turned it off recently,so go figure.
My 1985 Pontiac 6000STE had a C-QUAM AM stereo radio, and as I recall there were maybe one or two stations in my area (DC) at the time who transmitted in that format. Ain't hoid a woid about it since then.