Clinton was 97% favorite to win. They didn't HAVE to cheat, or so they thought. By the time Trump won, it was too late, they couldn't print up fake ballots or rework the software to give her the numbers. This time, they were so worried, that they OVER CHEATED and got caught because it was a panic cheat. They already had the software working for biden, but the 3% they plugged in wasn't enough, because Trump probably won by 10% or better, so they had to stop counting and haul in more ballots.
TL;DR- Dems were too overconfident in 2016. The end.
Easy answer to your questions:
Clinton was 97% favorite to win. They didn't HAVE to cheat, or so they thought. By the time Trump won, it was too late, they couldn't print up fake ballots or rework the software to give her the numbers. This time, they were so worried, that they OVER CHEATED and got caught because it was a panic cheat. They already had the software working for biden, but the 3% they plugged in wasn't enough, because Trump probably won by 10% or better, so they had to stop counting and haul in more ballots.
TL;DR- Dems were too overconfident in 2016. The end.