Here is a table showing the average of the polls in 2016 two weeks before the election vs the average now as reported by RealClearPolitics.
The first thing you will likely note is that Trump's standing in the 2016 and 2020 polls two weeks before the election in 2016 were not that different from the polls now. Nationally, he had 43% of the vote vs 42.4% now. In Florida, he had 45% of the vote vs 46.8 % now. It is the same with other battleground states.
So, what is different?
First, Biden is performing a lot better nationally and in the battleground states. His average is 51.3% now. Hillary was at 48.4%. Second, Biden is performing 2% to as much as 5% better than Clinton in the battleground states. Third, the Undecided or other votes (UD/OTHER) was much higher in 2016, double digits in 4 of the 6 battleground states. Those undecided voters swung largely to Trump in the final two weeks.
BOTTOM LINE: This time around, there just aren't that many undecided for Trump to pick up.
Read more about this comparison and the averages from FiveThirtyEight in 2016 vs now here; https://www.sfgate.com/politics/article/2020-polls-Joe-Biden-Trump-Hillary-Comey-Hunter-15655645.php