Depends how you’re using “why”. In Russian, they actually have two words for why, one of which implies teleology, and one which doesn’t, and merely requests some explanation for a phenomenon. I wish we had that in English.
In this case, it’s such a general question you can’t do much better, but you could, for example, talk about why oxygen-carrying proteins pretty much always incorporate an ion of something, in a merely cause-and-effect way. (And I actually don’t know the answer to that one)