Why would the Pakistan airspace closure cancel flights not headed to Pakistan itself? The India-Pakistan feud has heated up again, with Pakistan closing its airspace as a result.
I know many international routes fly over Pakistan, and that the closure would cause issues, but this report surprised me:

Thousands of people were also stranded by affected airlines that not only land in Pakistan, but fly over its airspace -- one of the major routes from Southeast Asia into Europe.
Thai Airways announced that all its European routes &quot;departing near midnight of 27 FEB through early 28 FEB&quot; were canceled &quot;due to sudden closure of Pakistani airspace as a result of tension between India and Pakistan.&quot;

Why would the airspace closure force flights over it to cancel entirely? I would think you would just reroute them around it.
To give an example of how flights can be affected by this in ways to make them impossible, Iranian airspace is closed from sunset to sunrise (unless things have changed recently).
Any aircraft that due to the closure of Pakistani airspace would need to cross Iranian airspace and be unable to do so because of that closure now needs to be cancelled or rescheduled.

Also, if the diversion around Pakistani airspace would stretch the endurance of an aircraft beyond what fuel it can carry, it cannot fly the route.

Or the diversion would cause such a delay in the schedule that it causes too much problems with the overall scheduling of the airline. Such things tend to have a ripple effect. One flight gets delayed by several hours, dozens of others get delayed as a direct result, hundreds more can suffer delays in the end.

Also, some airports only operate during daylight hours. A diversion around Pakistan might mean the flight can't make it to one such airports before sunset and thus can't land at its intended destination. Now the airline has 2 options, divert to another airport, arrange for bus or train transport for all the passengers, and in the morning yet another hop to the intended destination to pick up the new passengers, or cancel the flight.