I'm pretty sure the reason was that THFs and RNGs could stand at a distance against ranged attack monsters and evade it all until the monster decided to engage, which I think is forced at like 50% HP. Now you can always argue how balanced it is to be guaranteed to deal 50% of a monsters life without risk of death.

Of course with sleep bolts being around, you could practically kill a mob completely without it ever moving. So balance is already a bit so-so regarding ranged attacks.