What was NIST’s reason to switch naming from MD… (Message Digest) to SHA… (Secure Hashing Algorithm)?
When NIST introduced SHA-0 in 1993, they – for the first time – switched their naming convention from MD-n to SHA-n


Actually, MD-n was not NIST's naming conventions; it was RSA Security's (a private company) naming convention.  Before SHA (which was the original name; SHA-0 is retroactive terminology given to distinguish the original proposal from what was finally adopted: SHA-1), NIST didn't define any hash functions.