[MattBearman]

And bcrypt is better than sha512, why use an inferior option when you don't have to? bcrypt both exists and is free.

[16s]

Many are forced to use insecure hashing for compatibility reasons with outside vendors. Google email for orgs/colleges has two options for hash exchange (or used too... it may be different now) MD5 and SHA1. So you could not migrate user accounts unless the hashes were MD5 or SHA1.