Because Biden is a weak and uncompelling candidate who is only likely to become President at this late date because his opponent is so obviously a clinical narcissist borderline sociopath 12 year old dress up as a grown man.
In fact, the 2020 election is in some ways, or so it seems to a Brit looking in, to be mirroring what happened in 2016, only magnified many times: a less than compelling Democratic candidate with a good deal of baggage and manifestly unpopular amongst the working-class up against a staggeringly unqualified and ridiculous Republican one. In the latter instance, the less than compelling Dem candidate was good enough but not quite, won the popular vote, but lost the electoral college, and many people didn't realise just how bad Trump was.
This time, the Democrats are likely to have luck on their side: COVID has exposed the worst of Trump (what my sainted Mum would have called an ill wind that blew some good), and this time, despite a very weak candidate, the Democrats are likely to pull it off.
For what its worth, I've spoken before about many ties to friends in America in the Tri-State area: they report myriad Trump signs on lawns in upscale communities like Old Tappan, Harrington Park, Rivervale, Closter, and Westwood (NJ); Greenwich (CT); well-heeled Westchester suburbs; and towns in Rockland County. To be sure, they report plenty of Biden-Harris lawn signs, as well.
But the point is, these are not redneck flyover areas.
I am fairly convinced, not to say hopeful, that Biden will take it, so this terrible period of American politics can being recovery. But it won't mean, from what I can tell, that Americans are terribly enthused about Biden, only that another four years of Trump will be catastrophic for the nation.
And that is why Biden is not 20 points ahead of Trump everywhere.