i forgot where i read this, but WoW is a bit of a special case because several issues were just circumvented through game design.
example 1: there's no collision detection between players, i.e. you can block neither PCs nor NPCs. players blocking each other wouldn't have been possible with TCP at all.
example 2: you don't aim and fire and hit/miss shots due to timing and precision (twitch) - you select a target and execute an action which is sent to the server which will roll a die to see the outcome. it was fairly usual for the early game to break down when a lot of players congregated in a small area (hogger or tarren mill, anyone?), where an instant spell took several seconds to execute and PCs were teleporting around (after running into a wall for several seconds) constantly because their positioning updates took too long. but because of the style of the gameplay this still worked well enough in most situations.
what i want to say is that wow worked around TCPs performance issues by adjusting the game design. i can't fathom the reason for this - it might have to do with their focus on low hardware and connectivity requirements (for countries with unreliable connections). but even then i don't see why TCP would have been the better choice.