So how did they create such an effective art? They took the foundational techniques they had learned, worked their asses off, recruited a bunch of students, taught them, sparred a bunch in class, went out and competed (under various rule sets) against representatives of other martial arts, got into a bunch of fights, had a bunch of kids (like seriously - a whole lot of kids), taught them from an early age, had those kids work their asses off and spar and compete and get into fights, all along swiping any useful ideas and techniques they could find from other systems.
(Just to be clear, the Gracies were not the only ones contributing to the development of BJJ. However they were a hugely important part of it. Without their efforts the art would not have had the worldwide success it has enjoyed.)