Don't worry about the 'why', just accept the way it is. That's how collocations work. 'Forth and back' sounds very strange. 'Back and forth' sounds fine. Btw, what's with the 'ain't?' I know your English is of a decent enough level to know that this is a controversial word in English, people don't write it unless they're trying to imitate speech. Serious question, have you ever seen a native speaker, from any country, use it on this website? I would wager no. We just don't use it when we're writing normally, it sticks out a lot.