Why is it that...?

But the two proposals rely on the same evidence. The believer uses the strength of the evidence for natural laws to say that any exception to them would be a miracle. The sceptic uses the strength of the evidence for natural laws to argue they cannot be overruled. Both rely on the same evidence for their conclusion. The difference between them is a presumption about whether there is something that can overrule natural laws or not and Hume doesn't address this presumption in his argument.

One can imagine a conversation like this:
Martha: Jesus brought Lazarus back from the dead! It's a miracle!
Mary: Don't be silly, people don't rise from the dead.
Martha: I know, that's what makes it a miracle!