I have just learned about the Bibframe Profiles.

Since Bibframe is based on RDF, why are Bibframe Profiles not based on RDF?

As I understand, the constraints described in Bibframe Profiles are
constraints on RDF elements and vocabularies, and do not constrain mere
data. I'm not sure about what is meant with "structural constraints", maybe
also integrity constraints?

So expressing Bibframe Profiles in RDF as rules would be more beneficial to
the semantic web community.  I think it is possible to express the rules as
an ontology. By doing this, informal notations or plain JSON or EBNF
notations would no longer be necessary to express a Bibframe Profile, the
document could be rewritten to use RDF (serialized in Turtle, JSON-LD, etc.
whatever is convenient)