Generally it’s because of Playability or a Character on the card. I know Oracle features Umbreon so I assume that is part of the draw to the card (other than the fantastic artwork).
I don’t know anything about playability but I know Fishermans first print was in Skyridge and it’s still in modern sets so I assume that is part of the draw on that one.
Oracle was one of the best cards for the format it was in, with most decks draw engines based around it, so people building old decks (there’s quite a few!) are hunting them down, especially reverse holos.
Scratchdesk probably hit the nail on the head with Fisherman too.