"Standard HTML" is sort of an oxymoron.
Yes, you can do a lot of what's done on the web in "standard" HTML - but then you have to wrangle it into every "standard" browser, which turns out to be subtly different and full of bugs compared to the next.
It's not even possible to point the finger straight at Microsoft any more - Firefox has its fair share of bugs and an awful lot of non-standard DOM extensions, and every browser disables and enables a certain feature which the next supports. Support is even added and removed in certain browser sub-patchlevels and revisions - for example Safari suddenly started supporting certain DOM load events in a random security patch due to a merge of upstream WebKit.
Flash provides a common platform on which layout, interpretation, and feature support is similar (nearly identical) across all browsers on most platforms, something no other web programming solution can do.
Unfortunately that common platform isn't very good, but the homogeneity it allows is the continuing, and probably lasting appeal.