I'd say it's because this stuff relies on the person being cancelled and their facilitators accepting they were in the wrong. Boris, Trump, the Tories, the Republicans and their supporters don't.
I think I've mentioned it before, but Bill Burr pointed this out about Trump right at the beginning of his term. I dunno if it's exactly how it went down, but he used the example of a journalist pulling him up on comments he'd made about women and him responding "Nah, nah. I just said that about Rosie O'Donnell", them pushing him on it saying "No. You made comments about other women too though... " and him then going "Meh. You're probably right" and that was it. They couldn't really do anything because he refused to accept it was an issue. It only works if the targets cave to public pressure. See: Cummings. They just waited it out and now it's more or less over, he's still in power and everyone's moved on.