Probably because it isn't officially sanctioned, which apparently was the reason the more common libcs never added support for bcrypt directly.

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"The security departments of some customers look at recommendations they get and evaluate the deployed systems based on this. In a few places this led to the problem that the NIST warns people indiscriminately about the use of MD5..." (Note, this was from 2007 when md5 would be in roughly the same state that sha1 is now in.)

http://www.akkadia.org/drepper/sha-crypt.html https://access.redhat.com/articles/1519843#ok-ok-but-why-not...

SHA-2 is more widely supported and the de-facto standard these days. I'm not surprised that an entity like Microsoft ends up being conservative with its choices of algorithms. I guess they could've gone for SHA-3 but I suppose even that is a bit too "bleeding edge" for MS.

Microsoft dual signs at the moment with SHA1 and 2. SHA1 is used by older versions of Windows and SHA2 by newer versions. This flips a switch so older versions of Windows use SHA2 too.

Because the different hash functions have different trade-offs.

For example not all hash functions are cryptographic hash functions and can be much faster.