The Adeptus Astartes engineered the gene-seed technology at the First Founding of the 30th millenium, and since that time the technology to engineer new gene-seed or even implant Astartes organs has been lost. The rampant mutations of the gene-seed has caused a depletion in the supply of stable genetic material for culturing all 19 implants. Essentially, these factors mean space marine gene-seed is no longer a renewable resource and they must do everything possible to preserve it.
Now also consider that at the First Founding 20 different space marine legions were created, but each legion developed their own gene-seed technology independently. As a result, the gene-seeds were not intercompatible. Each type of gene-seed required a unique ritual to create a transhuman Antares. For example, the Blood Angels required the rite of Insanguination, while the Imperial Fists required the Hand of Faith. Thus just having a gene-seed was not enough, you had to have the right ritual for the right gene-seed.
This is why the gene-seed of each transhuman Astartes is vital, and for certain legions whose numbers are dwindling, those Astartes' gene-seed must be preserved at all cost. Once one legion is lost, it is lost forever. And if their numbers get too low to fight, well, game over.
Some Astartes have a more stable gene-seed than others, and this may be the case with the captain you are referring to. Specifically, the Primaris Space Marines' gene-seed has a mere .001% chance for suffering a genetic deviation between generations and can sustain even ionizing radiation much better than typical gene-seed. These Primaris Space Marines are also highly protected, as their gene-seed was derived directly from the Primarch.
Most of this is available on the Wikia page if you want more info.