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.