SHA-256 is faster when hardware accelerated, and was judged to be a more conservative choice. You can find their evaluation here: https://public-inbox.org/git/20180609224913.GC38834@genre.cr...
SHA256 with hardware acceleration might be competitive with BLAKE2, but not with BLAKE3 (performance graph at https://raw.githubusercontent.com/BLAKE3-team/BLAKE3/master/... ). BLAKE3 is also capable of being heavily parallelized without changing its output, as well as quickly updated without rehashing entire objects (if you keep some partial hashes around). Overall, it'd be quite desirable.
Hopefully all the work to abstract the choice of hash algorithm will make it relatively straightforward to prototype the use of other hashes.
I would really like an AES-based hash function as AES-NI is much faster compared to chacha20. There was a function called echo that worked like that during the SHA-3 competition but it was dropped before the final round.