Hume was an empiricist, so his argument about miracles is about the content of our experience, not our metaphysical speculations about its causes. If we saw miracles happen everyday, they wouldn't be miracles by definition because a miracle is a violation of natural law, which means it's necessarily rare. For just that reason, because of the nature of supposed miracles, we're psychologically geared to prefer a naturalistic explanation of a supposed miracle, because that's the type of explanation that reflects the bulk of our experience.

Naturalistic explanations will always be more likely because they're more empirically grounded; that is, they reflect more of human experience, which is crucial to the empiricist.

That's Hume's criticism of miracle claims: we're epistemically conservative in that we prefer explanations based on what's familiar to us, and miracles are necessarily, by definition, unfamiliar.