People built apps in Flash because they could put great experiences onto whatever outdated crappy IE version they had to target. There was also a really great e

People built apps in Flash because they could put great experiences onto whatever outdated crappy IE version they had to target. There was also a really great editor. Lastly, the APIs for a lot of stuff just didn't exist. There aren't really great wasm editors any more. JS frontend development has replaced most of that. Wasm is used but mainly to port existing applications to the web, like in this instance, or to implement client side computation heavy applications. It's also rarer because it's still quite recent compared to JS which has tons of inertia. Generally, incumbents are only replaced if the new solution is really better than the incumbent (or there is a powerful enough trend).