Fact is, we don't know exactly what the causes are of spontaneous remission in cancer patients. Science is being applied to try and figure it out as understanding how this happens may lead to a cure.
Of course, like so many things we don't know, many people are quick to explain away the unknown with miracles, magic, superstition and other supernatural forces. This is something humanity has done since the dawn of its existence.
People have attributed rain, thunder, volcano eruptions, earthquakes, tsunamis, etc, to supernatural forces ... and somehow continue to do so despite science having explained these natural phenomena.
The problem with explaining things away as miracles is that it doesn't actually answer any questions and isn't in any way useful to us. We can't reliably invoke miracles in order to cure cancer patients. The miracle doesn't explain how the cancer is removed. Basically, it's a useless non-explanation that cannot be tested or proven.