Well depending on why it was delayed 2 hours, that might actually have been correct at the time. I'm going to guess that by stage length, that flight probably can't have much more than a 2 hour delay before the crew will time out (won't be legal for how long the flight is). If it was already running 2 hours late for weather upline and had a high speed RTO, flight ops knew they wouldn't be able to re-crew it that late at night. It was probably already set to a WX delay, so that became the cancellation reason, until it was reported that it was mechanical and changed.

Above insight is from my experience working for UA. If we had IAD-EZE delayed more than say 70 minutes, our crew would timeout and they'd usually delay it until the following morning. The nice thing about US-Deep S. America flights is that the aircraft sits down there all day, so they really just have a delay on the US-SA flight. Coming back will have minimal, if any, delay. Since flights from US-Deep SA flights usually leave late at night, there aren't reserve crews readily available to re-crew them (especially not on a WX day that's probably already depleted them). That's why it becomes a morning flight. IAD-EZE is a slightly longer flight (763 back then), but I'd imagine the challenge for DL ATL-EZE is the same.