I'd say you can do anything with java-script and a canvas as flash could, but the number of people who could do it in flash was much higher than the number of people who can do it with "modern" tools.
This is the answer. Flash was at its best, in my opinion, with its low-to-no-code animation approach. Newgrounds is or was full of people creating splines-based, animated artwork in a way I haven't seen as easily and fully replicated by as many people as it was in the days of Flash.
I've been thinking about this sometimes, I don't understand why there hasn't been any editor suite to generate HTML5 + JS animations like Macromedia Flash did. On the foundation it's basically the same concepts: a canvas that can draw vectors and a scripting language based on ECMAScript. The editor then has a timeline control and can render the appropriate assets for animating it: vectors and scripts that animate those vectors based on the timeline information.
Is there some technical limitation I haven't figured out? Or is it simply a matter that no one has done it yet?
There is no technical limitation, it's very possible today. Commercial developers don't want to touch it because Adobe could sue them and/or it could be hard to extract enough money from such system, open-source developers apparently don't care enough to start doing it from scratch
Video killed the flash vector star and games moved to steam.
A good example of the former is a e-card service that eventually chose to move to video. Video is worse for their product but there was/is a more robust path to move to video than modern vector options.