The SACP have been riding the ANC's shirt-tails for the simple reason that it allows them to punch above their weight. It was clever strategy to remain relevant in a world that was celebrating the fall of communism as a failed ideology that oppressed its people to such an extent that they risked life and limb to escape to the "evil capitalist west". 
That said, at least they don't make election promises which a grade 8 student with an F in accountancy could prove are impossible to implement like some parties I know. You know as well as I do that the EFF's list of minimum wages stuck up all over town during the last election would result in massive retrenchments - especially amongst the poorest sector of our society! 
The EFF's election success thus far is wholly and entirely down to their ability to lie through their teeth selling hope to the desperate. (The parallels that can be drawn with the rise of crazy religious cults are just too obvious to ignore!)