Because the British government wants Russia for British business companies. That is why Americans, particularly those in Congress, are the biggest target for British anti-Russia propaganda; the British want to tamp down competition from Americans for Russian business.
That also explains why the fiction about Trump's collusion with the Kremlin was concocted in Britain and pushed hard in the U.S. by the British and their surrogates. Trump's statements about Russia early during his presidential campaign suggested that he wanted to facilitate as many business deals in Russia for American companies as he could. And his actions even before the campaign indicated his pro-Russia business stance. So Trump had to be hamstrung by any means necessary on the chance he became president of the United States.
I don't think there was anything blind about the Clinton campaign's use of the Trump-Kremlin collusion meme -- I'd say Hillary Clinton always knew it was a fiction. But the pro-Clinton American media blindly regurgitated the British propaganda as did the Democratic National Party, which Clinton controlled during her 2016 run for the presidency.
The blindness extended to the American branch of the 'Get Russia' crowd, which was rooted in the Cold War and continued even after the USSR dissolved. One would think they'd have put two and two together when they noticed that for all the British demonizing of Russians, the British wanted to do big business there. But hatred has a way of blinding people to the obvious.