The reason, to my understanding, that you don't teach rein back earlier on in a dressage horses career is to properly teach the movement of the rein back. It's not as simple as a horse just backing up, but lifting themselves through the back, with the ability to easily propel themselves forward out of it.
Also, I've found it harder to teach a forward halt on a horse that already knows how to back. Some horses connect the dots of "if I stop, human may ask me to back up" and they'll take a step(s) back once stopped (which is a deduction in dressage). We want them to be forward through the transition, and stop balanced, squared. If they are thinking backwards at all, it won't be a perfect halt.
Also, I've found it harder to teach a forward halt on a horse that already knows how to back. Some horses connect the dots of "if I stop, human may ask me to back up" and they'll take a step(s) back once stopped (which is a deduction in dressage). We want them to be forward through the transition, and stop balanced, squared. If they are thinking backwards at all, it won't be a perfect halt.