From what I have seen, the reasons why gssapi is used when kerberos is available are
* the Kerberos API is absolutely attrocious for common use, unless you're very keen on the low level stuff. gss is simpler
* most krb5 implementations come with gssapi anyway, and the concepts map well between the two
* the krb5 C API moves too much. The schism between mit and heimdal krb5 apis is an enormous driver of people towards gssapi.
* microsoft has built-in api called SSPI which is basically GSSAPI in disguise (wire compatible, but uses different named functions) and it uses their krb5 implementation ("active directory") as well as snego.
