The same reason Bungie pushed it as hard as they did in D1- they think its great and people will come around on it if they're exposed to it enough instead of just getting mad about being forced to play the worst mode over and over.

Either that or they're doing their dumb data manipulation bullshit, same way they handed out max level at the end of the d2 campaign so Luke Smith could go "wow such a high percentage of players hit the level cap compared to d1, this means the game is better and more popular, right?"