The Fastly approach has some advantages for wasm, but Cloudflare have mentioned that the vast majority of their users want JavaScript and TypeScript. Cloudflare's approach is more optimized for that: running a JS VM directly will take a lot less overhead than one compiled to wasm atm (since JS VMs depend heavily on JITing, and compiled JS VMs will have a much larger memory footprint due to the bundled compiled runtime).

It's possible that Cloudflare and Fastly users are different enough that different approaches make sense.