Argon2 with SHA-256 instead of Blake2 Can SHA-256 be substituted for Blake2 in the Argon2 algorithm?

I am working my way through the C reference source code for Argon2 and hand-translating it to Go. I was just wondering why Blake2 was chosen instead a well-tested algorithm like SHA-256.
If you take a look at the Password Hashing Competiton, you can see, that most of the schemes use Blake2b, some of them uses SHA-512, none of them uses SHA-256.

Blake2b is optimized for 64-bit platforms and this property fits exactly the requirement of a password hashing scheme. SHA-512 would also be OK, but SHA-256 would be much slower in software and the due to the fact, that SHA-256 is used for Bitcoin mining, custom hardware for SHA-256 is very cheap – this is exactly, what we do not want for a password hashing scheme. 

I do not recommand to substitute any part of Argon2. All cryptoanalysis refer to the original version. Argon2 is – unlike Catena (another scheme from the password hashing competition) – not a flexible framework, that has been designed to be used with variable algorithms. 
Argon2 uses not only the Blake2b function, but also something like a reduced version of Blake2b, the compression function G. You could possibly replace Blake2b by SHA-256, but you can‘t replace easily the reduced version, because Blake2b uses fewer rounds than SHA-256.