I would wager that DL does not fully test its systems for high availability and disaster recovery, and possibly not even standard performance testing (stress, peak load) in a production-sized environment. These are standard tech industry practices, that were also, in my experience, practiced in several other "critical" industries (in my career -- retail banking and investment banking).

The travel industry is of course not nearly as lucrative as technology nor finance. So I am not surprised that their technology investment is not as robust. But the impact of a major outage is clearly just as harrowing, and I suspect the regulatory agencies will continue to hound the airlines to QA their technology platforms.