As a note, while a few people may have denied the 2016 election results (as people have denied EVERY election result), the general popular claims regarding the 2016 election and 2020 election are drastically different in both what was claimed to be done and the remedy being sought.

First, let's establish what people are claiming for 2020. For the 2020 election, the people common things people are claiming is: "people illegally voted" "new ballots were created for people who didn't exist", "people voted multiple times", and "votes were lost/changed". The loser refused to concede the loss. And to counter this blatant cheating, people wanted the election fully overturned.

Meanwhile, there is the 2016 election. The claim was different. It was that Russia was interfering in the election, but not that the votes were changed/added/removed. And to my knowledge the remedy was not a new election or to overturn the results, but to hold Russia accountable so they wouldn't do it again and shore up election results. Hillary conceded that she lost the election.

These may sound different, but the severity and remedies involved are drastically different.

The severity of claims in 2016 is "They interfered to help the other side, and the results likely changed as a result of this". This is bad, but as a country it's a "ok...we should stop that in the future" level of bad. In 2020, the claim is "They literally stole the vote illegally so the results aren't what were counted, but rather a different result." This is REALLY REALLY bad. Like, if this is what happened, you can't trust anything about the government.

TL;DR: in 2016 people said "influence happened, we should stop that in the future" In 2020 people said "votes were changed. Burn it down."