I think I found why you cannot bind the keys.
You will not be able to bind hotkeys if you try binding any of the buttons W, A, S, D or Esc to any of the following functions: Pause, In-Game Chat, Evasion, Weapon Skill, Potion Skill, Use/Grab/Revive, Attack or Class Skill.
The solution is to NOT use WASD or Esc for any hotkey binds.