The rationale for this decision is discussed here: https://mailarchive.ietf.org/arch/msg/ietf-announce/FXpz3-UE...

An excerpt:

"For both symbolic reasons (that the technical position then is the technical position now) and to better ensure that IETF specifications reflect the spirit of RFC1984, a number of participants in the discussion felt it would be advantageous to recognize the substantive content of RFC1984 as a BCP."

Also, the RFC number is worth emphasizing.