It was like the JavaScript of UX and 2D games for a while. It was trash as a technology, but it was a technology that millions of UI designers and indie game developers cut their teeth on and already knew deeply, so it stuck around much longer than it should have. Like grown-ups trying to ride tricycles to work.
I remember someone at EA talked to Adobe about getting access to the Flash (.fla) file format so that our UI tools could read it directly to export into the in game format instead of reading the already compiled and compressed .swf format. And they were like, "Uh... you really don't want to do that. It's like a memory dump of the entire application state. It's a giant mess. We're really sorry."
By all accounts, it seemed like the Flash codebase was a dumpster fire. But so many amazing little games and UIs were built using it.