Why I can't transfer a Nincada using Pokemon HOME?

Question

I know that Nincada is obtainable in Pokemon Sw/Sh and BD/SP. However, Pokemon HOME won't allow me to transfer a Nincada that originated from BD/SP into Sw/Sh, and Pokemon HOME also won't allow me to transfer a Nincada that originated from Pokemon Bank/GO/Sw/Sh into BD/SP.

Why does this happen? Is there anything to do with the Pokemon's compatibility or something?

Answer 1

Nincada originally from Brilliant Diamond and Shining Pearl cannot be transferred into Pokémon Sword and Shield, and Nincada from outside of Brilliant Diamond and Shining Pearl cannot be transferred into those games.

This is likely a redundant protection against several cloning glitches due to Brilliant Diamond and Shining Pearl's "PokeDupeChecker" having cases to exclude natural clones such as Ninjask/Shedinja, and Pokémon originating from other games from being flagged and subsequently restricted from trades, HOME, and the Pokémon Nursery.

Pokemon HOME (Bulbapedia)