YouTube needed some extensions to HTML5 before moving to it as its default. Even then, it may still support Flash for old versions of browsers that many people continue to use.
The extensions were a variable-bitrate one and an encryption one. The first one is good for adjusting to different connection speeds and sets of playback hardware. The second one is good for implementing Digital Restrictions Management, er, Digital Rights Management (DRM).
Returning to Steve Jobs's famous
Thoughts on Flash, I'll do a blow-by-blow analysis.
1. Openness. I'm a bit cynical about that, I must concede. But Apple has been reasonably good at embracing open online standards, at least so far and at least so it seems to me.
2. The Full Web. SJ's argument was that Flash was superfluous for playing online video. While that was mostly hypothetical back then, YouTube and others have moved that closer to reality.
3. Reliability, security, and performance.
4. Battery life.
This suggests (1) negligent management by Adobe or (2) architectural flaws in Flash that are difficult to fix.
It's worth mentioning that Flash on Android did not last long, likely for these performance reasons.
5. User interface: "Touch".
Apple had once faced a similar problem with Cocoa, whose user-interface widgets are designed for keyboards and mice. The result: Cocoa Touch. SJ was claiming that there was no good Flash analogue to Cocoa Touch's smartphone-friendly user-interface widgets. Seems like another Adobe management problem.
6. Not making full use of iOS.
That seems a bit like platform protectionism to me, though SJ did have a point about the inadequacies of app-framework middleware.