Former Flash game developer here. A big part of it was artists and coders could use the same tool. An artist could create some flash animations and then I as a coder would go in and hook up all the code to make it functional without having to spend any extra effort to make the graphics or animation work (in other platforms you'd have to pack sprite sheets, color encoding, sometimes fight memory (at least I did on iPhone 4 back in the day), and then you'd have to set up all the animations manually and get each one working.

With Flash I was just sent an FLA, browsed through all the animations, went "awesome", and started setting up hitboxes and dropping them into the game.

Like I've wanted to port some of my Flash games to HTML5, but the intense frame by frame animation would be a bear to export and hook up in other formats.

And non-frame-by-frame animation was easy enough that I could help fill in the gaps when needed, no big deal. I even made a few simple animations just for the hell of it back then since it was so easy to work with.

I eventually migrated to console dev with XNA, then mobile dev with iOS, then some 3D development with Unity, and I've dabbled with Monogame, Phaser, Java, Pico-8, PlayDate, Unreal Engine, and currently working on a game in Unity again, since it's pretty much your only choice right now besides Unreal if you want good VR support. But yeah, nothing I've worked with has felt quite as nice as Flash as far as synergy between art and code, especially 2D art.

Examples:

I teamed up with an artist to make a 7 character head to head fighting game: https://www.newgrounds.com/portal/view/187047

I teamed up with another artist to make a richly animated (espcially the boss) short platformer.: https://www.newgrounds.com/portal/view/116067