Most of the fish that most readily get ich are fish that normally swim far, as in several hundred feet) from the sandy bottom. Fish that live on or burrow in the sandbed in shallower water are far more resistent, and slimier. Ich spends part of its life cycle in the sandbed. It's not catastrophic in the wild. In our tanks, it gets a chance at a population that has fewer defenses. As a hobbyist, if you don't take artificial precautions to keep it out of your tank your chances are totally that: chance, luck, and whether or not you bought from a shop and distributor that already apply precautions, as many of the large operations do. Understand that people are taking precautions for you, and that nature has some of its own.
The fewer times you add a fish to your tank---the less often you risk getting the parasite.
The longer the parasite goes without feeding and breeding, the less likely you have any surviving in your tank, whether encysted or not. Some of us don't add fish every year. This is, ideally, a case of getting fish you like, being sure they're ich free, and keeping those fish in a healthy tank that doesn't change much except in its surplus corals.
The fewer times you add a fish to your tank---the less often you risk getting the parasite.
The longer the parasite goes without feeding and breeding, the less likely you have any surviving in your tank, whether encysted or not. Some of us don't add fish every year. This is, ideally, a case of getting fish you like, being sure they're ich free, and keeping those fish in a healthy tank that doesn't change much except in its surplus corals.