In my limited understanding, a cancellation wouldn't be guaranteed to propagate to all nodes.

A feature of distributed networks. Kind of like it's not possible to delete Nostr notes - some relays would receive the deletion, some wouldn't, so there is no surefire way to delete it. A note only needs to be broadcasted to some relays, whereas a deletion would have to be broadcast to all, which is a much harder distributed computing problem.

With RBF you can effectively achieve a cancellation by sending it back to an address you control, but it makes the protocol more robust than a deletion would (because the double spend prevention with the longest chain heuristic, which is already in place, gives you the reliability needed without having to handle a special 'cancellation' case) and it costs a fee, which also helps prevent spam.