The phrase back and forth is in contemporary language nothing but the more oft-heard rendering of that age-old expression hither and thither. In both pairs, the paired elements are indicating first motion towards one’s current place and then motion away from it, and in that order.
You might just as well ask why the idiom is to say here and there rather than the other way around. By the same measure, why is it urbi et orbi and not its reverse — or coming and going, arrivals and departures, and even in and out? That is, why is it that we invariably name our current place first in all these expressions, these idioms, these fossilized phrases that have come down to us intact and inseparable, and irreversible?
There can be no definitive answer to why frozen idioms came to gel in one shape and not another, but perhaps here it is because of some predisposition or inclination towards thinking of the world from the perspective of one’s present location within that outer world. If it happened to Copernicus, it can happen to anybody.
In contrast, when speaking of voyages and journeys, the emphasis is usually reversed, because it is the destination that is of primary importance, the trip itself. That’s why The Hobbit has There and Back Again, meaning “going there and coming back home again”. It’s why a round-trip ticket (that is, a “return” ticket in the UK) in Spanish is an ida y vuelta ticket: going and returning. In both those cases, now it’s the other place that is the more important of the two, not the current one as we saw in the earlier cases.