1. UK’s has one major airport to get out of the country—Heathrow. Gatwick and that lot don’t carry the same weight. When Heathrow goes down, you’re proper stuck. Atlanta has DC, Miami right there.
2. UK allows transit visas, so half the people transiting can’t even step out the terminal, what do they do when the airport is closed?
The US doesn’t allow that, everyone clears customs/passport control, so no ‘no man’s land’ limbo for stranded passengers.
3. Heathrow's outage is going to take 24 hours as of right now. That's twice Atlanta