Because why just get Expansion sales when you can force people to grind through the entire story and rake in that sub-money.
They know that Dark Knight is a highly desired class, why would they just hand it out willy-nilly?

Still can't believe that they're tying in 2.0 into 3.0 though, gotta love having everything gated off even though you paid for it. All those newbies going through the ropes, means they're going to continue forcing us to repeat older content to get new things as well, like the roulettes and the relic grind. Oh-boy...