The issue is: obvious to whom? Both Newton and Leibniz independently invented* Calculus at about the same time. That may mean Calculus was obvious to Newton and Leibniz (and it may very well have been obvious to anyone in that particular community of mathematician), but it certainly wasn't obvious to the average mathematician at the time.

* More like discovered (don't want to imply math is patentable), but the point remains the same.