Because you can have mediocre to bad story but the gameplay can carry it. If you hate the story but still love the character building, stat work, gearing, combat, etc it can still be a good game to you. But for a lot of people, if the story is amazing, but the gameplay is bad, they'll just full stop.
Now in the case of the OP, he liked neither so obviously the game isn't for him, but even if the game has the best story in the world but isn't fun to play, I'm still not going to play it, you're better off watching the story on a youtube playthrough.
Which really boils down to, in 99% of games and genres, gameplay matters just as much, if not more then story, this is the same with RPGs.