It's different because there are already enough classes and races available, and already were even without Monk, Warforged, Hork, and Helf.
But DDO's prestige enhancements haven't yet reached that point of baseline completeness, and with their slow progress it doesn't seem they're even close. If all classes already had two decent prestige specialties up to tier 3, then maybe they could consider diverting resources to special paid specialties. But since prestige specialties are a key component of a classes's capability, to start doing it now would mean reneging on supplying the classes people have already supposedly been granted.