IE8 and Android 2.4 don't support it for one, but it does seem SVG has never really had traction. I've heard SVG has poorer rendering performance in most browsers (I'm not actually sure if that's true), and that some folks feel it makes it easier for their assets to be copied.
SVG is gaining traction, eg through use of d3.js. svgweb can work as an IE shim. Old Android is a problem, but on the other side Microsoft are doing a good job and pushing SVG in IE9+.
That has always confused me a bit. It seems that most of the stuff that people are doing with canvas requires writing a vector graphics layers on top of it anyway.
