My guess is the airline didn't have a process to approve landing at another airport.

Landing fees, storage, maintenance, and next flights on the schedule would all have been affected.

Plane probably usually flies back and forth already.

An admin looked at the schedule and realised they had to cancel the return flight already. They don't need the plane there it'll fly back empty.

Then there's the cost of landing at a different airport and unloading passengers, luggage etc. transporting them all onwards to the right place.

It's not difficult to see that to return the plane and passengers to their departure lounge and cancel the outgoing flight too was the cheapest option.