Why Thunder Wave can't hit Ground-types?

Question

It's status move, so why it cannot hit Ground-types?

Answer 1

Immunity is based on the moves type, not damaging vs. non damaging moves. The same reason Toxic has no effect on Steel types, as they are immune to poison (the type, not the condition.)

Comments

But Confuse Ray can hit Normal-types, cannot it?

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That is true, but remember that confusion is a secondary status effect, and thunder wave and toxic are main status effects (they show an icon when your pokemon has it.) secondary status effects are fair game for hitting types, like you can use attract on a ghost type despite it being normal.

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Also, give people a chance to explain before voting them down. :/

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I was waiting, NOW I will vote upXPP

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Weird, I was just going to ask this question.

Answer 2

Thunder Wave is the only status move that takes the type chart into account.
Consequently, it is the only status move that cannot hit Ground-type Pokémon (even when other status moves are affected by Electrify).

source
ie. Thunder wave is just programmed differently from other status moves, like confuse ray. I don't know why the programmers did this.

Comments

Probably for balancing reasons, since paralysis greatly cripples many sweepers, and a lot of support Pokémon get Thunder Wave. They’ve been slowly nerfing paralysis over the past few Gens, like changing the speed cut from 25% to 50% and lowering TWave’s accuracy down in Gen 7.

Answer 3

Thunder Wave is a status move which involves a weak charge which paralyses the foe.

Ruby/Sapphire/Emerald - A weak jolt of electricity that paralyzes the foe.

This is the explanation given by db here.
Ground types don't get affected by electricity. So T-Wave won't affect the Ground Types.