I expect that Trump will win re-election, and have thought so for quite a while. Here are just some of the reasons, in no particular order, that no one should be surprised. Each item is a comparison of the 2016 election with the current election. The below is by no means an exhaustive list; I don't have time to add the improved economic and social condition of all minorities, incuding women; what has actually been a very good record on Coronavirus policy (see Did Trump Take Coronavirus Seriously Enough?); the fact that the US now has effective enforcement of our borders, etc. But here are some biggies none the less.

Issue  2016 Election 2020 Election
 "Shy" Voters Trump and his supporters were teased, called names and verbally belittled. Under risk of being laughed at, these shy voters were enough to swing the election for Trump. For 4 years Trump has been, quite literally, persecuted. His supporters have been called racists, xenophobes, Hitlerian, fascists, etc. At least 3 Trump supports have been shot DEAD,  murdered in cold blood in the street for the crime of being Trump supports. The "shy" vote will be many times greater in 2020, because the incentive to hide support for Trump is so much greater. In 2016 you could be embarassed; today, you could be killed.  
 "Proud" Voters Probably not a factor in 2016.  By Proud voters I mean the radical Biden voter, typically young, extreme, often violent. They proclaim loudly to anyone that will listen - certainly including pollsters - that they will vote for Biden (aka Harris, Sanders, et. al.) because "Trump must be defeated". But will they show up at the polls? Young voters are notoriously unreliable in this regard. And attending protests makes them feel that they are accomplishing something; I believe these thugs actually feel heroic for their violence and intimidation. The simple act of voting provides no adrenaline rush or illusion of moral superiority. Besides, they have busy, busy schedules. (Those buildings won't burn themselves.)
Democrat / "progressive" hypocrisy The political right, a la Tea Party, etc. certainly recognized this in 2016, but it was still widely minimized and seen as not really out of control. During the Kavanaugh trial most observers realized that Democrats not only engineered slander, but were perfectly willing to ruin a man's reputation on zero evidence of wrongdoing - but on the strength of one witness, whose every verifiable statement was disproved, Adding to the ridiculousness of the hypocrisy was the claim that we should "believe all women" - despite the fact that roughly 50% of the liars in the world are women.  The Russian conspiracy hoax was roundly and repeatedly disproven, but Democrats to this day continue to imply that Trump is a Russian sympathizer - completely ignoring his record on Russian relations. Nadler & Schiff both claimed to have "hard" evidence of collusion, Schiff even waving an envelope to imply he had it in hand, and claimed it was "not circumstantial". Apparently he forgot to deliver this evidence during both the Mueller hearings and the impeachment trial or (as is obviously the case) he's a transparent, pathological liar. And the entire Democrat political machine is a co-conspirator in this amoral hypocrisy.
Foreign policy The Iran deal and NAFTA were largely seen as positives for the Democrats. But North Korea was seen as a volatile and perhaps inevitably a violent aggressor. What happened to all the wars? North Korea has not kept it's end of the agreement (let's all act surprised) but it's also no longer making threats. Our relationship is, by any reasonably objective standard, vastly safer and more stable. Iran is mad as hell - which has to be a good thing - but also for the most part "standing down" in the face of a President who knows the meaning and purpose of a "red line". Isis controlled territory has been reduced to zero square feet, and troops are coming home from Afghanistan. In the Middle East, Israel is signing diplomatic agreements with a handful of previously hostile or non-cooperating countries. Trump has been nominated for at least 3 Nobel Peace Prizes for these deals.
Democrat candidate corruption Hillary Clinton was widely believed/known to be corrupt in, at the very least, the political sense. But her worst documented behavior was setting up a personal email server, which was minimized by the press and seldom, if ever, correctly characterized as a crime. Joe Biden and members of his immediate family were under investigation by the FBI on suspicion of money laundering even before Hunter's laptop surfaced. Informed voters know that the laptop has been been verified both technically and by eye witnesses, who have given sworn testimony. The nature of these crimes is not just political, and not just the act of a money-grubber - they're blatantly unpatriotic and, I believe, treasonous. They are indisputable violations of Biden's oath of office. And even low information voters (see Are You A "Low Information" Voter?) have become at least dimly aware of these issues.
Frivolous impeachment Possibly some residual resentment over the impeachment of Bill Clinton which was, to a large extent, political. While acquitted by the Senate, he was cited for contempt of court. The judge stated, "Simply put, the president's deposition testimony ... was intentionally false". His law license was suspended for 5 years. His behavior was widely recognized as immoral. Very few would argue that his impeachment was groundless, although it was indeed mostly a political act. Trump was impeached and investigated over a phone call which according to Democrats needed to be investigated, even though a transcript of the call was immediately available. Every single "hostile" witness was asked (by Jim Jordan and/or others) if they had first hand knowledge of any crime. Each and every one answered "No", although some did not have the courage to do so directly. After being acquitted, the Democrats never mentioned it again. Not once during the Democrat National Convention. Not once during the Biden campaign. Not once during any debate or, as far as I'm aware, Town Hall. They hid it because not only was it a failure, it was a transparent political hit job. By hiding it now, they make obvious that not even they ever really believed it.
Trump as a "known quantity" In 2016 Donald Trump was elected largely as a way of giving the "middle finger" to establishment politics and it's practitioners on both sides of the aisle. People voted for Trump while crossing their fingers and hoping he would do more good than harm, but needing to tell politicians the status quo was no longer acceptable, regardless of the risk of electing an unknown. Desperate for change, especially after being sold "Hope & Change" and just being given more of the same, people voted for Trump as the lesser of two evils and hoped for the best.

Today Trump runs on a record that puts him near the top of any list of Presidential accomplishments, in areas as diverse as:

  • the economy
  • foreign policy
  • racial justice
  • work ethic

etc.