Nobody has mentioned the abandonment of SVG 1.2 as a factor. It was dropped by the W3C and never implemented in any browsers. This not only stalled improvements to SVG 1.1 but created a situation where the editing software (e.g. Inkscape) allows the user to create files using SVG 1.2 features which are invalid in SVG 1.1.
The abandonment has resulted in much-needed improvements to SVG being delayed. The last full release of SVG was 1.1 which was in 2003, we won't see another full release until SVG 2 which is expected in 2014, over a decade later!
Never mind the fact the browser implementations of SVG are still riddled with bugs and none are complete.