Probably because poison is a much more common threat to PCs than sleep and charm. I bet that the small bonus traditionally given to elves against those threats rarely comes in games (how many things have charm and sleep? Ogre Mage used to have sleep, several fey had charm... probably an order of magnitude less than poison based threats!)
Cheers
Pretty much.
Many DMs use a lot of vermin, snakes, poison traps, and assassins.
Few use wizards, fey, or ogre magi/oni with sleep or charm person.
Sleep and Charm spells are mostly a PC weapon for most groups. Poison has been traditional a monster/enemy tool.
Frankly if i can go 5 levels with seeing a giant bug or reptile with poison, I would not be upset at all.