Constant velocites are a part of the laws of physics. Ever since the beginning, light, sound, and all components of the electromagnetic spectrum have had constant velocities. It doesn't take long for the Maker of the laws of physics to compute things. When spacetime was created, the Maker said "let 'C' be" and it was so.
However, it is interesting to note that gravity and matter all travel at varied speeds...why is this so?