How come I don't have all the tiers?

Everyone starts out being able to hire Apprentice Tier I - V and Journeyman Tier I mercenaries. The Journeyman Tier II - V tiers are unlocked as you complete the Seeds of Destruction progression quests:

Oceangreen = Tier II
Bloody Kithikor = Tier III
Field of Scale = Tier IV
Invasion of the Rathe = Tier V

(You can complete Rathe and skip the others to unlock Tier V)