Because all software at that point was unable to handle the new date format. Imagine if today, all computer systems had widespread issues at the same time, on the same day. The only reason nothing happened is because people did their jobs.
Hope this helps.
Not even close to all software. There was a broad mix of stuff that used 2-digit years that would have had problems with it, stuff that used 2-digit years where it wouldn’t really impact anything, and stuff that used 4-digit years and so wasn’t a problem.
However, if it drove any sort of critical infrastructure, it had to be audited just in case it fit in the first category.