Why did the old fisherman tell Jerome that he could not be a good fisherman?

Jerome was told that he'd never be a good fisherman because he lacked imagination.

"... the old hands told me that I should never be any real good at it, and advised me to give it up. They said that I was an extremely neat thrower, and that I seemed to have plenty of gumption for the thing, and quite enough constitutional laziness. But they were sure I should never make anything of a fisherman. I had not got sufficient imagination."