Why is the speed of light a limit? and why is it just $3×10^8\,\text{m/s}$? Why does light travel with speed $3×10^8\,\text{m/s}$? and why not more?
 A: I will repeat what I have said elsewhere on another "why" question:
Physics is not mathematics. It is an observational discipline that uses mathematical formulations to fit observations and predict the behavior of new set ups.
Experimental observations can be fitted very accurately by using quantum mechanics and electrodynamics where the velocity of light c is constant. This constant has been  measured and is an experimental fact.
The answer is  it travels at that speed "because this is what we have observed/measured".
In general, physics does not answer "why" questions , only how from certain assumptions and using mathematical formulations one can describe physical systems.
A: Let's take one of your comments:

So than this 3×10^8 os just an experimental observation , this mean its not right to say that nothing travel faster than speed of light . 

The main experimental observation by, say, the Michelson-Morley experiment, or other experimental confirmations of special relativity is not the speed of light's numerical value, but that this particular speed is invariant: it is observed to be the same for all inertial observers. As I show here and in the linked answers, the form of the Lorentz transformation can be derived from very basic physical assumptions that have nothing to do with light. Such derivations show that either the World must be governed by Galilean relativity, or by Lorentz transformations between inertial frames, but they do not tell us what the value of the special parameter $c$ is. The do, however, foretell this parameter's invariance between inertial frames. 
So, given the arguments I just alluded to, and given that experiments have shown the speed of light to have the invariance property, these experiments show that light happens to travel with this invariant speed $c$. One way of interpreting this experimental result is that light is mediated by a massless particle. Neutrinos were for many years thought to be massless and they are observed to travel at the same speed $c$ to within contemporary experimental uncertainty. They are only indirectly known to have a very small mass through the phenomenon of flavor oscillation. If the massless graviton of quantum gravity is shown to exist, it too must always move at the speed $c$ relative to any inertial observer.
But there is no explanation known as to why $c$ has this particular value. Everything I have said would be true if the invariant speed were $4\times10^8{\rm m\,s^{-1}}$ and light were observed to propagate at this speed.
