It's really very simple. The Dems and their many stooges in high places in the Obama administration (Comey, Brennan, Clapper, Rice, et. al) and at the FBI, counted on HRC winning, so that none of their illegal efforts in favor of one candidate over another would be exposed (right, mr. insurance policy, Peter Strzok?)
When that didn't happen, they jumped on the next best thing(s) which was Russian collusion. But how to sell something so absurd to the American public? Why would Putin favor Trump over Clinton, when he had spent the last eight years walking all over president Obama and HRC as secy of state? Do the Dems NOT remember how Obama just let Putin take Crimea, in spite of treaty obligations to defend Ukraine?
In order believe something so incredibly insane as Trump being Putin's b!tch, would require convincing the public that the Russian had something on Trump. Sure, they believed he was no good anyway, but only if Putin had leverage on him, would he be a puppet for Putin. That's where the Steel dossier comes in.
Well, without any proof (the dossier has proven, in Mueller's and Comey's own words, to not be reliable) they used their puppets in the media to spread the false narrative, and the rest is tragic history.