It’s not so much about the gates. It’s about which flights need customs and immigration and which ones don’t. A flight arriving from a domestic city can divert to any other airport and easily receive services, fuel, dispatch paperwork, employees to help deplane etc… They can let passengers off that airport to rebook them on other flights through other cities, etc. An international flight cannot land just at any airport and have those services or employees available. They can divert fairly easily into Houston or Dallas, but they sure don’t have as many options as a domestic airline have with service in every city in Texas. They become a bit of a bigger priority since they have fewer options. You've got crew rest legalities as well, a crew that originated overseas but diverted to a different airport may or may not be legal to takeoff again to make it to the original destination.
Looks like the airport is open now and accepting arrivals. Both flights coming from Heathrow today are showing 30 and 45 minutes delayed for this afternoon.