Now that I think about it; Chronivac’s evolution may be directly linked to sites like CYOC. People collaborating on an interactive story might be more likely to take it in new directions. In contrast, Master PC was relatively centralised*, and people wrote separate stories. The original story attracted people with particular preferences/fetishes. They write more stories in the same vein, which draws in more like-minded people.
*I had to go looking, and the index is still preserved on the Alt Sex Stories Text Repository (ASSTR). If you’re interested, I found the Index page here. The two hyperlinks in the first section (directly above the first story listing) give JR Parz’s inspirations, and his guidelines for writing your own Master PC stories.