Chameleons can change color to match the color of their surroundings. If a chameleon was born

unable to change color, why would it be unlikely to pass on this trait?

Since chameleons evolved to be able to change colour in order to camouflage themselves and avoid being eaten by predators, a chameleon unable to change colours would not survive very long. Because it would be easier to hunt and kill, it most likely will not live long enough to mate and pass on its genes.