Most species of Hechtia specialize in growing in the toughest, driest ecological niches as they can. By growing in places like this the plants minimize the potential competition with other plants. If nothing else can grow there, there is no way for something to either crowd them out or shade them out. Usually this means that they grow in the desert, in broken rock faces, or loose rocky soil.
This species is a bit different. These plants were growing on boulders and the faces of solid rock! No cracks where the roots could search down deep for water hidden far under the rocks.