Well, there was a good reason for that, for a long time: IE didn't even begin to support .svg until version 9, which means that it isn't a realistic option for deployment for anyone who needs to support pre-evergreen browsers.
I investigated them a year or two or so ago, because I found that AngularJS could work quite nicely within an .svg document, which opened up some exciting possibilities. My recollection is that at the time, there were some critical cross-browser problems with font rendering that made the topic very kludgy and complicated. I except that matters have improved since, but I do not know to what extent.
And IE still doesn't (nor do they plan to) support declarative animation in SVGs. (SMIL) Someone there decided that script based animation is more robust so they just bypassed that capability. I think it's a terrible idea because you forego having tightly integrated SVG animation modules that can just be dropped in as needed.
They've been around since 2001, but browser support only got good recently. Initially Adobe and Corel made browser plugins but Corel scaled back and Adobe bought Macromedia.
Also SVG GUI tools haven't been great. Inkscape is strongly print focussed, or at least was the last time I used it. There's no reason to set a page size on a web svg...
Someone needs to build an SVG equivalent of Flash Pro.
Nice, contained way to show data like this.