It is in FIPS 186-5 which is a very recent standard. Likely the underlying implementation has not undergone recertification to include it, as NIST has quite the backlog for FIPS certifications.

See: https://nvlpubs.nist.gov/nistpubs/FIPS/NIST.FIPS.186-5.pdf and https://csrc.nist.gov/projects/cryptographic-module-validati...