A voting system needs to be accurate but it also needs to inspire confidence. I think the U.S. failed on the latter point. Let me be clear that I don't think this affected the outcome, only the confidence in the outcome.

Mail-in voting has existed for a long time, but it is being done at a larger scale and in new ways and new places. It was a mistake to do it during such a contentious election. Covid is not a good enough reason: it's easy enough to drop the ballot in a county box so the secretary of state has a chain of custody for all valid ballots. I would actually prefer voter ID as well as people standing there to do a brief check. In addition to preventing fraud, it also helps remind voters if they forgot to sign or something.

Also, I think it needs to be said that a lot of people hate Trump. Hate can cause people to do things that they would never imagine doing otherwise, like election fraud. It doesn't need to be a conspiracy... just a lot of individuals doing small stuff (post office employees throwing away likely Trump ballots, etc.).

And it doesn't help that the map looked quite red on the day of the election, and that the poll numbers were WAY off, and that the media was rooting for Biden. It all just erodes confidence.