They compete on different use cases, most js/ts apps cannot be (reasonably) compiled to wasm and also wasm sandbox doesn't limit what the environment can do.
As a glaring strawman if you expose eval to wasm it will not help you.
Deno's sandbox will allow you to crate a dedicated worker with no network/disk access to handle sensitive informations, or to force your application to use a specific worker as a proxy (by making it the only thing with network acess)
Sandboxing doesn't sound so unique or innovative when WASM is coming along and doing the same thing, and with a much wider audience and thus more likely to have massive traction.