There is no other explanation, let's be honest, someone could just unlock these races through finishing the storyline and even then that would be too much, there is absolutely no reason to lock them, it doesn't make sense lore wise either, the rep farm for these particular factions is really really bad, world questing for several weeks on argus is a hell. If you got to the point where you actually managed to unlock them then the expansion would already have advanced enough and you will probably not want to start from zero lvl so you will either pay for a boost or a race change = more money for blizzard.