It is not worth serving a city if there is rocket fire reaching the city and perhaps the airport.
Israel certainly knew what they were dealing with when they went after Hamas in the West Bank. Israel's economy is heavily dependent on tourism.
They have to figure out the problem.
If US and UA do not divert, I'm not sure I would want to be a crew member responsible for bringing the plane back out or in.
Also, DL's statement noted that they were working with the FAA. This isn't just an airline decision w/o FAA involvement.