Probably because they're planning on converting lancer to have a secondary job that is a tank class. Currently Warrior is intended to be a damage and recovery tank, so from gladiator it gets the threat generation, and 20% more health from healing, from pugilist it gets an evasion boost, more health from healing, and either higher critical rate or a last-ditch self-heal. Although some of this could come from lancer too, it would probably be too much overlap when/if Knight gets added making neither class really stand out.