my opinion, since that’s the only opinion I have
Before we entered this era of close elections, we also had 50% turnout, but the Electoral College was all but forgotten. A 10 million vote difference was not uncommon. We had serious third-party candidates and debates were managed by the League of Women Voters. Then the two big parties got together and took care of that. Recently, we have had a rise in participation but it is still less than 2/3 of eligible voters. On my graph, that’s the top line; Voter Eligible Participation. 60 to 80 million are completely disengaged or disaffected, not even registered.
The close lines of those who actually vote for President, the ones at the bottom, tell me that the big parties don’t even try to draw from the half of the country that is telling them they aren’t doing their job by not voting for either one of them. Ralph Nader tried to make this point in 2000 and got unfairly blamed for Al Gore’s loss. Obama campaigned in rural districts and probably did bring in a few million new voters, but then the US went back to it’s usual level of apathy. Trump did something similar in 2016, but notice that his actual votes were roughly equivalent to the population increase for Republicans historically.
Was the real story of Hillary and Kamala the gender gap? I don’t know how to prove that, but I suspect it’s true. Biden had a similar bump to Obama, possibly due to the reaction to their predecessors. When things get bad enough, Democrats finally get their butts off the easy chair and go vote. But apathy sets in again, and add the class ceiling to that, and 10 million people stay home. That’s the current count, 81 million voted for Biden, 71 million for Harris.
Sources: Voter Turnout in Presidential Elections | The American Presidency Project
Wikipedia for Presidential vote counts