RFC 6979 - Why not simply hash the message & the private key for deterministic ECDSA?
Deterministic signatures are safe in the random oracle model. Using HMAC_DRBG allowed me to rely on existing research on the safety of that construction and how close it comes to a "true" random oracle. If I had used any other "handmade" construction, then I would have had to provide extensive analysis on why it is secure. Being naturally lazy, I chose HMAC_DRBG.

Moreover, this use of HMAC_DRBG makes the end result more "convincing" -- and a large part of specifying a cryptographic algorithm as a RFC is to get other people to use it.

(If I had been aware of its existence at that time, I might have considered HKDF instead of HMAC_DRBG, but that does not matter much in practice.)