Direction of stresses in torsion

[Vatsy]

In the fig. attached below,a circular bar is subjected to torsion. A point is considered on the surface of the bar(denoted by point 1). The shear stresses are shown .
Why don't we take one of the stresses in the radial direction??

 

[PhanthomJay]

The shear stresses vary linearly in the radial direction, from 0 at the center to a max of TR/J at the circumference , but the direction of those stresses on a planar element are circumferential and axial ( longitudinal) only.