Leibnitz was open about his discoveries with respect to Calculus, and his notation was easier to understand and to use.
Newton kept his work a secret, not wanting to reveal it until he had developed it to the fullest possible extent. Also his notation was more esoteric and obscure.
Leibnitz was an "open system," Newton was a "closed system."
IMO, even if Newton was first with the Calculus then Leibniz still contributed a great deal by creating a much superior notation.
Amazing how the time for a new idea comes along and somehow touches several people at about the same time..
I believe Claude Shannon was a man of similar brilliance in comparison to Newton and Leibniz...