The prologue is pretty much the exact opposite of what the rest of the game is. It's a dreary corridor solo hack and slash with scripted events everywhere, why would you want people to see your game like this?
Actually in my opinion the prologue is a perfect example of the game, and it demo's exactly what one should expect from start to end in TERA. Truth be told if i had played the prologue i would not have bothered to play any further. The entire game is in fact 'solo hack and slash with scripted events everywhere," i can pretty much draw a line from 1-60 which you follow with little to no deviation.