Because of PVP. The more build options you allow, the harder it becomes to balance the game. The Division had this problem, everytime Massive would try to re-balance the game, people would (and still do) come up with these realy strange, obnoxious builds that unbalance the game. Most of those builds were useless for playing the PVE portion of the game. They sole purpose was for gaining advantage and trolling people in PVP. Then Massive would have to hunt them down....nerf them...then the process would start all over. I suspect that Bungie will add more "attunements" as build options as the game grows.