I think it is partially genetic, but also the type of environment that Grothendieck was raised in. He was raised in utmost uncertainty during WW2 (he was half-jewish), and i think he gravitated towards the certainty in mathematics. He was also mostly self taught early on, so he definitely had a different way of looking at things than those who came from regular academic backgrounds like serre. I like to think of natural brain power as hardware (genetics) and the set of beliefs, interests, fundamental ways of thinking about the world as software acquired through experience. Even the best hardware can't run shitty algorithms. In any creative fields, there are an extraordinary amount of brute force paths, it takes a special type of touch to find the correct path, and this seems to be very much a function of the life experiences a person had and this is really what differentiates special creative people like einstein or grothendieck from people who are just as intelligent but haven't done nearly as much.