I guess just because it seemed fairly niche, seemingly targeted mainly towards people making 2D games for cross platform use, around the same time things like Unity & UE became the hot new thing to be using for that type of game dev. People also seemed to look down on it's AS3 like syntax and there was a limited pool of people willing to commit large numbers of hours to working on it as part of their career path when Apple were bringing out new frameworks every year for doing things like AR, 3D, SceneKit, IAP, etc. It never seemed it could fully embrace the newer native abilities that everybody wanted to be working with while maintaining its ability to cross compile. I guess browser based gaming (Flash, Facebook games, etc.) kind of died at the same time as well.
We used to do lots of Flex development back then for various commercial projects, then Flash/AS3/Flex got all but abandoned overnight, becoming almost hated. We managed a large cross compiled (iOS/Android/Windows/OS X) AS3/Flex project with native hooks for various platforms up until mid last year, parts of the codebase were nearly 10 years old, when we finally managed to convince the client it would cheaper to re-write in a mix of HTML/JS/Electon/JS bridges than continue to battle the existing codebase. We rewrote it all in around two weeks, improved the UI and fixed long term numerous issues that had plagued us for at least a couple of years, mainly due to being reliant on closed source 3rd party Windows/Mac integrations/hooks that had been abandoned by the original developers and Adobe also largely abandoning the AS3 iOS/Android compiler.
Also, it didn't really seem to have a huge community and everything seemed to be in a transitory phase with things like Flash being pushed out by iOS native development, the huge improvements in HTML/JS/CSS, etc. Then tech like Swift, React, etc. came along it just seemed to get forgotten as a tool in our arsenal.
We certainly started focusing more on other new stacks/approaches around the same time.
Because in terms of game engines, new engines that get much more publicity, like godot engine, exist. And as Haxe supported Flash when I played with it, I as well thought it was superceded by newer engines.
Half a decade is a lot in human years, let alone IT-years.
