Because it goes in that order. The first letter on the keyboard is Q, and then W and so on. Usually there is a setting on phones to change that, so it goes in ABC order instead of the regular QWERTY order.
I believe it is because the arrangement of the first six letters in the top alphabet row of a standard keyboard QWERTY goes way back. The layout was created in 1873 when Christopher Latham Sholes made improvements to his earlier typewriter models in order to design a quicker and more efficient typewriter for E. Remington and Sons.
