A lot of pokemon seem not to get moves that would seem to fit them perfectly, because “they would be too good with them.” For example, snorlax doesn’t get slack off. Giratina, the god of the distortion world, doesn’t get trick room. Zekrom, who literally shakes the ground when sent out, can’t learn earthquake. And several explicitly evil pokemon, like chandelure, can’t learn nasty plot. There are several more, but these annoy me the most. But I understand the game needs to be balanced. Thoughts?
I don’t think that is the reason, to be honest. I mean, if preventing Pokémon from being “too good” was ever a matter of even the slightest concern for Game Freak, then making Giratina and Zekrom available to players in the first place was their mistake, not any specific item in their respective movepools. Even if they’d given Zekrom Earthquake, Reshiram would still have been even stronger because Dragon/Fire is just such a potent combination under 5th-generation rules (i.e. with no Fairy-types). I’m much more inclined to suspect some obscure flavour-related reason for these absences – like, Snorlax is literally always slacking off anyway, so he really shouldn’t expect to get any special bonus for doing so more than usual. Or perhaps they were simply oversights; it simply didn’t occur to the designers to stick Nasty Plot on Chandelure for some reason (I mean, when we think about this stuff we tend only to give any thought to the 5 or 10% of all moves that are useful at the competitive level; they presumably give consideration to the whole lot, so you can see how they might forget things that seem like obvious choices to us). Either way, eh. I have a saying: Pokémon should be good at the things they’re good at. If it makes sense, let them have it.