In wet or icy conditions a permanent repair wouldn't actually work; the hot bitumen would instantly cool before adequate compaction could be achieved and the ice or water would also prevent the repair bonding to the road.
Permanent repairs take more time and, depending on their location, may require a road closure or temporary traffic lights to be brought in. This requires more equipment, staff and planning, particularly at times when the resource isn't available immediately - perhaps because highway staff are on winter gritting duty and have more urgent potholes to fix.
Potholes, or a series of them, may be symptomatic of a more general, underlying problem on the stretch of road. This requires further investigation and potential resurfacing of an entire road section. This is a larger job and so can't be done immediately.