I think that the best explanation for Fighting type attacks being not very effective against Poison type Pokemon is because many Fighting type attacks are based on something that involves physical contact. Touching anything that is poisonous is, in my opinion, maybe a little bit dangerous. I feel like Poison type attacks doing normal damage to Fighting type Pokemon doesn't really need an explanation. There's not really a reason, in my opinion for Poison type attacks to be super effective against Fighting type Pokemon, and there's not really a reason for Poison type attacks to be not very effective against Fighting type Pokemon, so Poison type attacks do normal damage against Fighting type Pokemon.
