What I don't understand about Evolution

I'm a pretty big fan of Charles Darwin (especially his awards show) and his theory of natural selection. (IIRC it is his Theory of Natural Selection that evolutionists use to bolster their case, but the theory of evolution is not specifically due to Darwin. Correct me if I'm wrong.)

In any event, I was watching a fascinating show called Planet Earth which I highly recommend if you can find it. (Below is an example of the sights you'll see on Planet Earth, but it is a home-made video from some fan of the show so the music is not part of the show).





One episode was about shallow water and they talked about a salamander in Japan (Dragon Salamander???) that was like 2 meters long. That's big. They explained that the salamander has poor eyesight and lives in murky, cold water, so it has developed (through evolution and natural selection) this phenomenal electrical impulse-measuring system that allows it to sense other prey (fish) as they come near based on the infinitely small electrical pulses the fish give off simply by being not dead.

Wow! That seems like a VERY HANDY adaptation that makes a lot of sense.

Except, why didn't the salamanders also evolve into salamanders with good eyesight? Its very confusing to me, if the salamanders did, indeed, evolve, why did only the most esoteric and one-purpose trait get better? Isn't good eyesight the least bit valuable?

Sheesh... very confusing to me.