Starting off, SVG is an XML encoding, which is a rather hairy thing to deal with. Just to get a feel for LOCs:
https://github.com/GNOME/libxml2
There's also the web's (horrible) tendency to make a diarrhea of special-purpose tags and attributes--many of which could be subsumed by basic turing-completeness. Compare this to postscript, which was a relatively simple stack machine to begin with.
And of course, if we start special-purposing things, we're no longer compliant with the standard, and it would be chaos to call it SVG at that point.
