Nobody postulated that the speed of light is constant because it was obvious: light just like every other wave has a constant speed in the medium though which i

Nobody postulated that the speed of light is constant because it was obvious: light just like every other wave has a constant speed in the medium though which it propagates. The fact that nothing can go faster is a consequence of the Lorentz transformation, so nothing had to be postulated. The important thing that Einstein recognized is that the Lorentz transformations apply for all physical phenomena and not only the Maxwell equations and that started a revolution on physics.