As RadioFan and Second Choice mention, radio listening in general is low after 6:00 PM. Listening to AM is even lower.
Second Choice also correctly mentions AM directional arrays are expensive to maintain, and many of them were built in areas that would've been considered exurban at the time only to find the city has crept up on them. That type of land is worth a fortune, probably more than the entire value of the station let alone nighttime billing, today.
Also, in the case of KGA, its former owners owned an AM on the same frequency in the San Francisco Bay area, too. I can't remember if it wanted to expand the nighttime coverage of the Bay Area station or if it wanted to add it to a daytimer, but it had to downgrade KGA to upgrade the Bay Area property. It was similar to what happened with WOWO in Ft. Wayne some 20 years earlier. WOWO downgraded its 50,000 watt nighttime signal so a new sister station in New York could be on-air 24/7.