Imagine you have to investigate a traffic collision and witnesses tells you “an unexplainable entity caused the collision with an incomprehensible force it used on both drivers.” Still you follow up just to be thorough you ask the witnesses if they can tell you anything more about this entity or force and they admit they can’t. I believe most people would realize the witnesses have not provided you with anything other than an untestable theory and certainly haven’t come close to explaining the accident because “the unknowable” has exactly zero explanatory power.
An explanation increases our understanding of a phenomena but appealing to the unknowable can’t do so because it just replaces one mystery with another. However substitute “unknowable” for a supernatural explanation like “leprechauns” and suddenly people seem to forget the fact that you’ve explained nothing at all. This despite the fact people readily acknowledge the supernatural to be inherently beyond human comprehension. The reason is because ascribing an action to an agent feels like an explanation even when it’s claimed that agent behaves in inexplicable ways and is itself incomprehensible. “Poseidon causes the tides” might seem coherent but all you are really doing is giving currently unknown forces a proper name.
To attribute the unknown to a supernatural cause is to both not illuminate the original issue and create a host of new questions. What is behind this force? What is this entity? If I were to claim that my television works because of imperceptible dancing leprechauns I could just as easily claim, without proof or possible refutation, that it was the singing of imperceptible gremlins. In fact there are infinite possible unknowable reasons for every phenomena so the appeal to any given unknowable can not enhance our understanding but only create the question of how you could isolate one unknowable force among the infinite possible unknowns. Moreover in order to appeal to an entity to explain something we must currently have some cognitive grasp on the explanation and so by definition unknowable entities can never qualify as an explanation.
There are many more factors involved in establishing a good explanation like simplicity, coherence with background knowledge and the successful prediction of new phenomena. However while many other factors contribute to explanations any given supernatural explanation could be substituted for “it was magic” or “I don’t know” and still retain the same explanatory power, that being none at all, and thus can never actually be an explanation.