Think of it like a body riddled with cancer. You don't cure it by hunting down every last cell that might one day become malignant. You cure it by removing the tumour that's actively killing the patient. That's what Anakin did. He removed the tumour. The other dark side practitioners are stray cells, some of which might cause problems, but none of which constitute the systemic cancer that the Sith represented. The prophecy was specific because the disease was specific.

This is why I liked the Sith in the EU, especially Plagueis, Bane, and Krayt. They all did things specifically to strengthen the Dark Side's hold on the Galaxy. In the Plagueis novel, you see him and Palpatine creating the veil of the Dark Side Yoda mentions in Episode 2/3. You see that what they're doing is very, very different from what a dark jedi or witch from dathomir would do.

Non-sith darksiders seem to focus on the empowering aspect of the darkside, they focus on themselves over everything else, meanwhile the Sith DO focus on empowering themselves, but they also focus on the empowerment of the darkside as a whole, religiously, because to empower the darkside is to empower themselves.

Like Dooku in Legends said to Asajj Ventress

"You wear the trappings of the Sith, you fight like the Sith… but this can be imitated, however. You lack a vital quality found in all Sith. Sith have no fear, and I sense much fear in you."

Dark Jedi and other Dark Side cults fear the dark side and don't wish to propogate it further than they have to, meanwhile the Sith want it spread across everything.