They have some services that are "global", ie not tied to a given region. Those services' requests are actually processed all over the place, but south central is a big datacenter. The 9th biggest in the world, apparently. When it lost cooling and shut down, everything routed around it as planned... But it caused so much extra traffic that it overwhelmed the connections to other datacenters. The backlog of requests is tremendous of course, so even after they got south central back up, all the other datacenters are way over their traffic capacity. They've got the Datacenter back up, and are now restoring storage and storage dependent services.
Honestly it's hard to imagine a good mitigation for this. "Build more datacenters" is already happening as fast as it can. "Keep enough spare capacity around to handle losing one of the biggest datacenters in the world" is pretty unreasonable.
If you, as a customer, are uptime focused enough that it's worth paying extra, then the sensible practice has always been Cross-Cloud infrastructhre/failovers. At least since the Amazon Easter failure of 2011. That's what giants like Netflix do.