I think this is just a matter of idiomatic usage. This NGram suggests that before long didn't really become established until early/mid C19, and many of the (relatively few) earlier instances involve extended forms such as "before long time be spent".
To my mind, all that's happened is that before long has become an established shortened form of before a long time [passes/has passed/had passed].
As it happens, the corresponding shortening didn't occur with after long. There's no special reason why it should have, and thus no reason to say this fundamentally differentiates before and after.
As John comments, long itself is often a Negative Polarity Item, which is why we can say "It didn't take long", but not *"It took long". Possibly (I'm guessing wildly here) this encouraged the uptake of before long (because "before" there effectively negates the long passage of time), whereas with after long you'd be affirming that passage of time.