There are a lot of answers here I partially agree with, but there's an overarching concept that I think they're missing: there's no market for it. Adobe never stopped working on such a tool, as others have pointed out below, in that they still ship Flash's successor Animate. They kept Flex going as long as they could, too.

I think the reason people aren't terribly aware of Animate, and why there isn't a competitor product—whether open source or commercial—with a big name is that we don't need/want it. I haven't looked at the numbers in a while, but last I checked most web traffic was coming from mobile devices, and heavy, high-gloss interactions akin to what Flash afforded and what could be accomplished with WASM/WebGL are not what people want in their mobile browsing experience. They want fast. WASM/WebGL may be pretty efficient, and may be the best way to accomplish the sorts of things Flash was used for in much more efficient ways, but they're still not the best way to render text with a few buttons and images (i.e. a typical web page). The use-cases that had us reaching for Flash in the "aughts" just don't come up very often in web development these days.

When users DO want heavy, high-gloss interactive content, they're going to the App Store/Google Play to find it, not the web. This gives the developers the choice of building those experiences with WASM/WebGL if they like, but the vast majority are choosing native SDKs or a game engines. Against those options the WASM/WebGL offering is just not competitive, yet (the APIs, tooling, community is just not as good, yet).