"Why can't Flash make use of Quicktime?"

Then you'd have to do version-checks, if you're actually delivering content to audiences. Fewer people have it, fewer people use the current version. Most codecs on content sites are H.264 or On2 VP6 (successor to Theora). Easier to solicit Apple to open up their acceleration APIs to plugins.

<em>"The simple fact remains that Flash is so resource-hungry that it is undeniably badly written."</em>

You're quite incorrect.