Delta was down for most of a week, while their competitors were up in a day, because they had no way to bring up all their computers in a way where they didn't immediately crash each other. The terminals would overwhelm the databases, and then the terminals would hang. Most airlines didn't have this problem, so as they got their computers booting again, the network slowly came back online and back in sync.
Southwest specifically updated their systems in the last decade because of a previous outage. The "they run Windows 3.1" memes were just memes.