A couple viable reasons:
Directional antennae cost a good deal more compared to Omni-directional ones. Granted, theirs is a basic carotid pattern which is fairly common and a basic “off-the-shelf” design.
If the antenna is 20-30 years old, especially in an environment with salty moisture in the air, it may be in need of replacement.
Considering radio is not a huge moneymaker like it was 20-30 years ago, considering the market they sell ads to is still covered with the downgraded coverage, they’re not as worried about losing 5-10 miles of city grade coverage in the country over spending $20-50k for a new directional antenna (plus installation)
Plus, they’re probably halving their transmitter power output, which isn’t an insignificant expense in small-market small-business radio…that’ll probably save them $2k/year in power usage!