The Ghost and Normal/Fighting immunities can be explained by the fact that normal people can't touch ghosts, and ghosts can't touch them; the ghost would go straight through them, so they can't harm each other. Fighting is similar in that punching a ghost would be just as ineffective, but their minds are more honed from training, so they have the power to sense, and as a result be hurt by, ghosts.
Grounds immunity to Electric is because the easiest way to dissipate an electrical charge is to ground it. This is why lightning rods on buildings are so effective, as it gives lightning a quick path to the ground, where it can't hold a charge, and as a result, dissipate.
Steel being immune to Poison is because Steel creatures are not inherently natural creatures; they're made from steel instead of flesh and bone. Because of this, their bodies work differently than organic creatures bodies, and so Poison attacks aren't as harmful to them as it is to us.
Flying being immune to Ground is because... well, when you're flying, you aren't on the ground, so it can't hurt you.
Dark being immune to Psychic is due to that the dark represents the unknown, because when you look out into the dark, you have no idea what could be lurking there. The unknown is also one of humanity's biggest fears, and when a mind is faced with the unknown, it completely panics and is effectively crippled.
Fairy being immune to Dragon is because in most Western fairy tales, fairies are or are typically aligned with the protagonist, and dragons are typically the antagonist. There are exceptions, but that's usually what ends up happening. And good always beats evil in fairy tales, so Dragon gets crippled against Fairy.
