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.
People level for lvl 10 to 50 doing flashpoints and still are oblivious as far as their role goes. Same for PvP.
But there are lots of people coming from other MMOs who know exactly what to do and/or are quick learners of new mechanics. They want to get to lvl 50 asap and I can't blame them - it gets tedious on your 5th+ alt.