Actually, in the episode of Bardock, Bardock becomes the so called Legendary Super Sayian that was talked about in legends and myths throughout Sayian history (I dont want to be a spoiler so Im not gonna tell how this happens)
So my guess its a genetical potential that all Sayians and therefore saiyan/ *insert species* hybrids have, but you need a crazy ammount of training and will to do it. Most Sayians turned became Super on Earth, where the Sayian were completly different from the ones on Planet Vegeta
(my personal guess is that you need some kind of "humanity" if you want to activate it, and since Sayians were a ruthless warrior race, only the few sayian survivors who got to earth and gained a different perspective on emotions were able to do it - Goku, being raised as an human from the start did it first than Vegeta for instance, and their kids did it even sooner - Bardock only did it [[ again huge spoiler]]] after fighting Freeza in defense of Goku's life and spending some time with the Plant aliens and seeing the kid get beat up)
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