Because it's not good for the game.
Even without rhe 2xXP weekends, it's already way too easy to level in this game and there's really too many people that reach 50 without understanding how to play PvE in a group or do warzones. The more you simplifie leveling, the less people understand their class, their role and how to behave in a group.
Such a silly argument.
#1 this game is not very hard to begin with
#2 almost every healer that I know & most tanks leveled as a DPS. If they are competent gamers they do just fine in their end game roles.