There are many, including:
Four more years of demographic changes. America has literally become less white since 2016. And, as is naturally the case, more of Trump's older voters have died in the last four years than have Clinton's 2016 voters. Voters who are eligible to participate in their first presidential election in 2020 strongly favor Biden. Trump is not running against a candidate who the Republican Party has spent 28 years demonizing. Joe Biden is not Hillary Clinton, and a relatively ineffective six month campaign to sour Americans on him is trivial compared to what Hillary Clinton had been subjected to for decades.
The Democratic Party is more unified, and has been so for many months, heading into the 2020 election than it was when heading into the 2016 election. Democrats control the House of Representatives. That means that adverse news about the Trump Administration receives national attention. Recent case in point, public scrutiny of sabotage of the U.S. Post Office.
U.S. media (with the exception of FOX which remains a constant) long ago stopped falling all over themselves to give wall to wall coverage stressing Trump's "unique maverick appeal" to voters, including hundreds of millions of dollars devoted to free, non fact checked coverage of virtually all of his campaign rallies
Trump now has an actual track record in office. He no longer gets to be all things to all people dissatisfied with the status quo in America. People can no longer simply project their hopes for how Trump will perform in office onto him, now America has seen what a Trump presidency actually looks like.
Trump faces organized significant opposition to his run for reelection from many figures who were previously well established in Republican and/or Conservative circles; various Never Trumpers, the Lincoln Project: Republican Voters for Trump, previous members of other Republican Administrations etc. "Establishment" Republicans mostly muted their criticism of Trump once he secured the 2016 nomination. Now a significant number are openly attacking him and promoting Biden to restore sanity to Government. They hold little to no sway over Trump's base, but they can effect the election at the margins, among some Independents, moderate Republicans etc.
Third Party candidates are in eclipse. We acknowledge to role the Green Party played in Trump's narrow victory in 2016. The Green Party has a significantly reduced profile heading into November. But it is the loss in stature of the Libertarian Party that is more note worthy. Libertarians far out polled the Greens in 2016. Libertarians appealed to significant numbers of younger voters in 2016 with their "pro-pot" position and anti-establishment branding etc.
There is an African American candidate on the Democratic ticket this year. There wasn't in 2016 when we lost, there was in 2008 and 2012 when we won. African American voter turn out dropped in 2016 from the prior two elections. That hurt Democrats.
Trump no longer has the element of surprise going for him, and nowhere is that more apparent than in the Midwest, where the Clinton campaign counted their chickens before they hatched. It was inconceivable, back in pre-pandemic days, for a major party candidate to skip campaigning personally in any contested state, let alone one with ten electoral votes. But Hillary Clinton did not make a single personal appearance inside Wisconsin after she won the 2016 presidential nomination, and she wasn’t a frequent visitor to some of the other previously blue Midwest states that Trump ended up carrying either. Trump’s degree of Midwestern appeal caught the Clinton team flat footed. That is a error that the Biden team is not making this time around.
The pandemic will have killed a quarter of a million Americans by the time election day arrives. This is on Trump's watch, and all of his pronouncements about how perfectly his administration has handled this catastrophe are only playing to his hard core base, his approval ratings on this front are damning.
The public is unhappy to a record degree over the course our nation is on. That is almost universally a referendum on the incumbent Administration. Trump can spin all he wants but everyone knows that the economy remains in terrible shape, largely due to how poorly the pandemic has been handled.
How horrific a human being Donald Trump is has been well litigated and firmly established a thousand times over with floods of new information now available about how utterly loathsome a human being he actually is. No, this does not negatively impact on his floor of support. What it does is lower the ceiling of support beyond his base that Trump can still aspire to, and his base alone doesn't cut it.
Of course there are things that Trump can and will play up to make sure that voters who are naturally inclined to support someone with his professed world view show up at the polls for him. Even Barry Goldwater got almost 39% of the vote in 1964. Any major party candidate in America starts out with over a third of the vote locked in solidly. And those voters tend to be fervent and vocal. I had a friend (of above average intellect, honest) who was convinced George McGovern was going to win in 1972 because where he lived in Oregon at the time McGovern was wildly popular.
Trump can and will rally his core base. He will win back almost all of those who might have started wavering in their support of him. But his core base is not enough. No doubt Trump will identify and turn out for him this year some rural white voters who failed to vote in 2016. But the converse is true for Democrats also, except more so. Far too many Democratic leaning voters assumed that Hillary would win without then last time. Their sense of complacency has long since been shattered.
Yes, Trump has racism and his specter of "Anarchists" rioting in our cities to galvanize his voters in 2020. He had racism and the specter of the M13 gang infiltrating suburbia galvanizing his voters in 2016, and just barely squeaked by then. Trump trotted out the specter of drug, disease, and crime ridden caravans of illegal immigrants invading our southern border to galvanize voters in 2018, and that cost Republicans control of the House of Representatives. Meanwhile Mexico still hasn’t paid for his wall.
Trump will have Russia "meddling in our elections" on his behalf this year. That was true in 2016 also. But now it is out in the open, and some social media platforms have actually become at least somewhat more active in combating Russian efforts.
Election fundamentals have shifted AWAY from Trump since 2016. Grassroots Democrats are far more organized now than we had been prior to the 2016 election. Trump’s election was an eye opener of the first degree, and the immediate response to that came the day after his election with a nation wide (and beyond) mobilization of women against him. Republicans have lost ground nationally in every election year since 2016, and Democrats historically turn out the most in presidential election years.
In my opinion constantly repeating the refrain that this will be a very close election only helps give Trump the cover he needs to more blatantly attempt to steal it. If conventional wisdom hardens on a prediction that this can go either way, then it becomes easier for Trump to after the fact claim that he was the legitimate winner, but for rigging, once he is defeated.