Wanna know the real reason? C is just the maximum floating point value of the computer that our universe's simulation is running on. Similarly, the Planck length is the value closest to zero.
I reasoned about it in the same vein but with a different approach. We are running in a discrete simulation. The most you can move is one space quantum per time tick. That speed is C.
More likely, it is the maximum speed the physics engine can handle per tick.
That's exactly my point. If the speed (= distance / tick) is represented by a float, then we can travel no faster than the max float value.