Because if everything is on one button there is no possibility of screwing up and the classes would play even more easy than they already do. Going 1 -> x / 2 -> x / 3 -> x while using off gcd´s, defense cd´s and moving around is way more complicated than just pressing 6 times "1".
The tanks already have not the most complex rotations and you guys still scream for more easy mode. Do you really want another DRK situation? I already fear what they will do to the GNB in the next expansion, especially after what they did to the HW DRK.