Why international flights dont get cancelled?

Some international flights do get canceled. However there are several reasons why they're given prioirity to fly in poor weather. International flights are usually on larger, heavier aircraft that can cope much better with bad weather. And canceling an international flight is much more disruptive to the system - e.g. if a flight from the US to Europe is canceled then its return flight has to be canceled too, and very often there is no way to get those passengers where they're going until days later. Whereas with domestic flights there is usually a way to shuffle planes and seats around to make things up. So what few takeoff and landing slots are open during bad weather usually go to the long haul flights.