Because removing the slime coat on fish when handling them reduces their survial rate upon releasing. It is their protection against bacteria in the water from killing them. Science indicates that survival rates for released fish are about 82% with barbed hooks and proper handling, but fall to 25% when handled by those have no desire to learn anything that does not benefit just themselves at the current instant. Contact any local conservation agent or biologist and they will tell you the same thing.
And yeah Lamprey, share the knowledge but really, pay for a product ad if you are trying to profit. This forum could use some money for the bandwith and servers it uses and it is not a personal flea market. Try a sponsor ad.