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 ;).