What does this do that a .bib file doesn't? It doesn't follow a standard which is made clear even by the creators themselves: "When you put a CITATION.cff file in the default branch of your GitHub repository, it is automatically linked from the repository landing page, and the citation information is rendered on the repository page, and also provided as BibTeX snippet which users can simply copy!"
I don't see why they don't stick to the established standard and parse a CITATION.bib instead. It would be less complex, more friendly to the user, and less likely to cause lock-in.
I guess the answer is semantics: who will guarantee (e.g. to downstream services) a CITATION.bib file will contain the metadata for the software in the repo? CFF is single-purpose and made for just that.
The guarantee is that you have citation information for a specific research output type: software (or dataset, as defined), and that it is the output you have found the CFF file with. Unless people want to break the principle on purpose, against which no format/mechanism can do anything ;).
I don't see why they don't stick to the established standard and parse a CITATION.bib instead. It would be less complex, more friendly to the user, and less likely to cause lock-in.