Honestly I can't understand the mental calculus that went on in the heads of Adobe execs at the time. Yes, cleaning up the ball of mud that the Flash codebase had become and making it not so battery hungry wouldn't have been an easy task, but it would've futureproofed it significantly. Instead they decided to keep tacking on new features which ended up being entirely the wrong decision.
EDIT: The constant stream of zero-days certainly didn't help things either. A rewrite would've been worthwhile if only to get a handle on that.
The bugs were definitely Adobe's fault: as with most tech companies, they were far more interested in expanding the feature set than they were on fixing the bugs and stabilizing the platform.
The reason it burned CPU cycles is that non-coders could make programs with it and they would produce the world's worst code doing so that "worked". The runtime itself was fine (efficiency wise, not all the other things).
I haven't looked in a while, but I believe there's music and audio production tools with similar approaches.