Each airline has its own risk tolerance, some airlines continued flying to Bali during the volcanic eruptions where as others cancelled their flights. Knock on effect plays a big part in the decision making process, local airlines could simply turn back to base but foreign ones were looking a a diversion with hotel, crew rest requirements and the aircraft being unavailable for 12 hours.
Possibly crew or aircraft availability would have been lacking and any disruption would have thrown a big spanner into network scheduling with a greater adverse effect than a few cancellations.