HTML, SVG, CSS, JS, etc are all good and valid technologies that are missing just what made Flash -and internet- attractive to many designers: a creative oriented IDE (Interactive Development Environment).
More and more disciplines of professionals are coming to the web space. It wasn’t that long ago when one smart ‘webmaster’ could get a site up and running by herself. Now we specialize more and more: designers, developers, Ajax coders, CSS designers, PSD artists, studio people, Motion graphic talents and a long etcetera.
It’s about efficiency
Many very successful (and functional) sites have been created out of pure HTML with nothing more than text editors. That idea is not opposed to the growing need for pixel perfect, platform independent, engaging and highly interactive experience. That not to say that such an experience that can only be created with proprietary technologies such as Flash, Silverlight, Flex, etc. However, the rise and success of such tools represent the need for efficient environments to produce multimedia content for the web.
As of today, that production setup comes in the form of specialized IDEs, where form and content marry.