Why are miracles rare? And might they be rarer today than in past times?
Let's take as the working definition of miracle, an event that cannot reasonably be explained by natural causes or laws and that, in its context, gives substantial credibility to a claim that it is caused by God apart from natural causes, and/or to a claim that a person is speaking for God or doing God's will.
It is evident that such events are rare, at least today, in western society. Why are they?
One modern explanation, of course, is that miracles never happen, that apparent miracles are the result of misinterpreting events, and that since we now both understand natural causes better, and know better the limitations of our knowledge, it rarely happens that what can't (yet) be explained by natural causes can't simply be attributed most reasonably to chance and/or to the limitations of our knowledge.
But suppose we don't accept the premise that miracles never happen, and assume some genuine miracles occur. Why might they be a rare occurrence? At least two philosophical and two theological answers suggest themselves.
(1) Because it is self-contradictory for them to be frequent, as if they were frequent, they would necessarily have some overall determinate intelligible pattern, at least statistically — and that especially if they are to give credibility to a claim about their connection with God — and would thereby be part of the natural intelligible order of the universe, and in this sense explainable by natural laws.
(2) Because God places great value on the natural order, which would be undermined if things such as miracles that cannot be assigned a proper place IN that order were to occur frequently. And God places value on it because of its order, or intelligibility.
(3) Because of our lack of faith. (Cf. Mt 21:21, Mk 11:23, Lk 17:6)
(4) Because of man's lack of faith in general. (Cf. Mk 5:6-7, "He could not do any miracles there" and Mt 13:58 "He did not do many miracles there because of their lack of faith")
These two philosophical explanations for why miracles are rare correspond fairly closely to the most common philosophical arguments for why miracles are impossible. Noteworthy about them is that they both suggest that genuine miracles will be less frequent the better man understands nature. The better he understands nature, and sees patterns within it, the more easily would an event that was previously explained as due to a special divine intervention, yet still within the mysterious workings of the world, be now either assigned a place within the natural order, or seen as deeply contrary to the natural order.