How do people end up taking unusual antidepressants?

Suppose you rank all antidepressants by how much they're used. If you go down to like the 30th one on the list or whatever (sometimes you'll see ones that are only marketed in certain countries) then what would explain how people ended up taking it? They always start you on the first-line medications, so wouldn't it take a very long time to work your way down to the exotic and unknown ones?

And if anyone did work their way down and then find success, isn't that highly scientifically interesting? Shouldn't there be lots of literature about the topic of people finding success only with a particular antidepressant that's way down low in the rankings...since you could look at that molecule's mechanism of action and ask "How is it that this particular one helped resolve this person's depression but none of the more highly ranked ones did? What is it about this mechanism of action that might make it work for this patient?".