In the theory of relativity the upper bound of velocity is included by using the Lorentz transformation. Therefore the theory of relativity can give no answer to my why question. However, in my preprint I explain why:
Preprint On the upper bound of velocity
In theoretical electrical engineering you learn that the group velocity of electromagnetic waves in wave guides can not exceed c. The reason is the electromagnetic properties of empty space and the group velocity of electromagnetic waves. This is derived without needing the Lorentz transformation.