I am not aware that gravitons have been discovered yet, but I think they are expected to travel at the speed of light because a) they are massless and b) are predicted to have similar properties to the photon.
I assume you mean "gravitational" waves (because obviously not all waves travel at the speed of light) but again the reason would be that gravity and light share some properties, and one of them is speed. I am not a physicist so I assume that there is a better explanation...
