They are cancelling flights because those specific airports don’t have the equipment to provide a precision landing (ILS) during low visibility weather. They only published a landing procedure with GPS. They will now spend a few months installing the ILS system to enable such landings so they can resume commercial flights.

Before GPS, all airports would be either VFR only or would have an ILS system installed. Since then, some airports were built without an expensive ILS but used a published GPS approach to allow low-vis landings.

They could have continued flights with only VFR navigation, but that would seriously restrict the weather they could operate in.