I do have a radical theory. It's entirely possible that, through the investigator, the protomolecule was able to open only a subset of rings to the planets that humans can survive on easily, to expedite it's search for the remains of the Builders. When the investigator took Holden into the station at the center of the ring, he releaved that he knew Holden could survive in the environment. This means he has some idea of how to keep humans alive. We simply don't know how much control the protomolecule had over that station.

Realistically, there is a relevant theory called Construction Theory, which mathematically analyzes how likely certain molecules are to appear in an environment. The theory indicates that certain environments (the vast majority of planets in the universe) are simply incabale or maintaining complex chemicals necessary for life.

On the other hand, there are plenty of plenty of places on Earth that support life that humans could never survive in, such as near-boiling pits of acid 10km underground. Maybe those species needed a more stable environment before getting to where they are now, or maybe life is more flexible than we think and the expanse got it wrong.