Because people have short memories, and lying gets you attention. There's this resistance grifter named Jon Cooper who had "sources" saying Trump was almost certainly die of COVID, or that his arrest was imminent. He got tens of thousands of retweets from those claims. They turned out to be demonstrably false, but people still follow him religiously.
On some level these people know they're being lied to, they just like the way the lies make them feel. Poso is selling the idea that Antifa and the far left are an all consuming cabal that can get to any juror in any case, when they can't even get a witness who won't torpedo their case right out the gate. If your enemy is all powerful, you feel less stupid about worrying about them constantly.
That said, the way they covered this case in particular is maybe one of the most vile things I've seen, and I usually don't get too worked up about this kind of thing. I don't 100% blame people for wanting to believe this.