This is most likely due to how elements work in D&D. They're based on the "classical" alchemical elements fire/water/earth/air and mix with positive and negative poles to create the others, like electricity or cold. The system was complex but logical a few editions ago, and then it was revised a lot over the years which didn't help. As the name implies, these are natural physical forces from which everything else is built. If you would try to convince a DM/narrator, your best bet would probably be a Wujen from the eastern Realms, as they're using the RW traditional Chinese elements which also include wood, which stands for slow progressive growth, spring, the sour taste and of course plants. One could argue that poisonous plants and therefore poison are part of that domain.
