Krustlet implemented the Kubelet API rather than the containerd-shim API. That means that with containerd shims, you can use the default K8s kubelet, containerd, and extend your containerd runtime with a shim for executing Wasm. Kubelet is much higher in the stack and requires much more to implement it and everything below it in the stack.

Just FYI but krustlet is dead and the community has moved on to approaches where the container runtime handles the difference instead of having special nodes for it.