I think some people on the SVG design committee were aiming to replace Flash for things like browser games, and wanted animations and javascript and so on to support that role.

That lead to the weird situation where browsers have two ways of embedding an SVG into a web page - embed in an <img> tag and the javascript won't run, but embed it in an <iframe> and it will (but of course iframe height can't auto-size...)

The javascript also means pretty much no user-generated-content sites allow the upload of SVGs. Wikipedia is the only place I can think of - and even they serve the SVG as a PNG almost everywhere.