I think one way to think this through is to acknowledge that some religious people are superstitious, and others are not. One may even conceive of a superstitious atheist, e.g., one who has a lucky number, who senses an omen, etc.
Superstition is one instance of state of mind that has an affinity with religious thinking, i.e., the belief in another realm of reality that impacts the one of which we are normally conscious. Religions, as has been noted, typically map out that other reality, and this reality’s connection with it, in rather precise terms. Superstitions, as has been noted, typically take that state of one and apply to just a few states of affairs – a ladder, a black cat, whistling in a house, etc. In my religion (Judaism), there are some teachings highly critical of superstition, some teachings that are neutral, and some that rather encourage it. I consider my a religious person, but not superstitious.
Religions typically have communities that celebrate, worship, study together, have a canon, traditions of ethics, authoritative figures, or at least some process by which authority is understood. Superstitions require none of these.
Religion, for most, is a value neutral word – religion can be good or bad, when assessed by some outer, rational criterion.
Superstition carries with it a rather negative valence. It evokes a sense of magical thinking, irrationality.