so I was curious as to why the human race is (or seems to be) in charge of Babylon 5

IIRC, there might be more reasons given in "Sky full of Stars" and "Babylon Squared", so I won't spoil the reasons here (even though it's background information).

Needless to say...It actually makes sense why it happened that way. It's more than just the multicultural humanity thing, but it also plays off humanity's previous experience with aliens. (Humans have a naivete problem, oh yes.)