The rich all in one tooling is one part, but the packaging, and sandboxed nature of them is what spawned the ecosystem.
An author could just focus on their experience, without getting bogged down in any Web complications (or indeed requiring any web specific knowledge at all), compile out an .swf, and share a simple file. Those hosting could then easily embed them knowing they were completely confined to their little box, and couldn't escape.
Similar Solution here
(What's nuts is that all the Web developers don't get this. They end up writing all their tooling against NodeJS, instead of the JS runtime everyone already has and that, for that reason, is the entire basis for how their field is actually able to produce things that users can actually use.)