Its weird that the Grass type has never had a cover Legendary. Not sure if Calyrex counts, since its the mascot of the Expansion, not of a box. Even if it does, this would be the first time in more than 20 years that Grass has had a legendary mascot. In fact, Virizion was the first Grass type Legendary ( Not Counting Mythicals ) and that was in Gen 5. Its so strange for a being starter type, you think it would have more prominence, but no, its kind of like how scarce Fire is a type despite being starter type.
I honestly think its bizarre that Grass hasn't had a Cover Legendary. Lore wise, there are countless nature deities and Forest Spirits from various cultures and religions, so I don't think inspiration is problem. Looking back it, Xerneas would be perfect Grass type, it sleeps in the form of a tree, and can create forests. Yet, no Grass type.
If I had to guess, the reason we've hadn't had a Grass-type cover legendary is just because the themes they go for never lead to the Mascot Legendaries being Grass... except Xerneas totally should have been a Fairy/Grass (if I had to guess why they didn't, they wanted to make Fairy "look good" for its first Legendary; however making it Fairy/Grass would have put it on even ground with its counterpart's Dark/Flying). Another issues if probably a Grass Legendary being a nature spirits which they made in Celebi which is a Mythical... and then again with Shaymin which is also a Mythical.
Psychic I have no explanation for. In the base game Bede is the only Psychic specialist and he doesn't stick with that the whole game, switching to Fairy. There's Avery but he's exclusive to the DLC. Feels like an odd omission, no matter how you slice it.
It feels like every generation there's one Type they can of let fall to the wayside for no reason. Last Gen it was Ice-type having no Type Expert unless you count Sina.