Disclaimer: everything that follows is a personal opinion - not an assertion of fact.
NIST has created Federal Information Processing Standard (FIPS) 140-2: Security Requirements for Cryptographic Modules. FIPS requirements are mandatory for Federal Government agencies, as prescribed by FISMA law. FIPS-140-2 Annex D covers Approved Key Establishment Techniques. The only FIPS-approved password-based key derivation algorithm is PBKDF2 (NIST SP800-132). Login.gov uses scrypt, which is not FIPS-approved. The FIPS-approved key-derivation algorithms are mostly covered by NIST SP800-108. Login.gov uses several custom approaches for key derivation, none of which are FIPS-approved.