Flash filled an real need; for a lightweight portable graphics
programming environment, and for an ubiquitous reliable video
codec. That, plus a lot of determined marketing by Adobe, got us
to the status quo, where it’s assumed that every computer has a
Flash player installed.
One more thing: Flash Studio offered developer tools that let designer/developers who were (and are) more designer than developer create software. That’s where Flash still beats HTML5; you need to be more developer than designer to create Flash-like things using HTML5. For now. There’s a real opportunity today for an HTML5 IDE for designer/developers.