Watch this video and ask yourself how many BBC reporters, journalists, editors etc know any Trump supporters, Brexit voters or even Conservatives.
The disconnect between the people and the media and opinion formers is huge in the UK and the USA. The disconnect means that the mainstream media including the BBC can't see what's happening in the country. If all your friends, family and colleagues support Hillary Clinton, want to stay in the EU and vote Labour, Lib Dem, SNP or Green then why would you believe that other views have much support.
Thankfully this disconnect has happened at a time when people don't have to consume the news that the mainstream media including the BBC pump out. The Internet has made alternative views easier to find and people are finding out what the mainstream media including the BBC are hiding from them. The BBC barely reported on the latest Wikileaks revelations but people found them on YouTube etc and educated themselves.