Every linux box in existence supports PS, very well. So does every PDF reader. That covers over 80% of all computers. Extending this support could be done through a plugin. Compare this to the fact that IE does not support SVG and PS is a far better standard to work with in terms of vector drawing.

As to the turing completeness of SVG, how efficiently can JS transform vectors in comparison to PS. Not very fast.

Also, nobody wants to draw just paths.