Implementing something new in JS can be done relatively easily using a slow path, where you just write some privileged JS or C++ and then wrap it, without doing any optimizations. Then if it gets popular the vendors can optimize it at their own pace.

Implementing a new feature in WebAssembly is a bit more complex due to its execution model and security constraints. I expect it's also just the case that a lot of these new WASM features are very complex - promise integration is super nontrivial to get right, so are WebAssembly GC and SIMD.