Here's why:
* Nasty Plot is a move learned by leveling up. It's not a move passed down genetically like egg moves.
* Togetic and Raichu are different species. They do not share any common ancestry.
* Moves are not inherited in Pokémon breeding. Offspring inherit a small pool of moves from their parents, but these are typically egg moves, which are specific moves determined by the parents' species.
Even if Raichu knew Nasty Plot, the offspring wouldn't automatically inherit it. The offspring would have to learn it by leveling up.