LDAP can be preferred over X.500 because:

A.
LDAP is a protocol to access Directories and X.500 is not

B.
LDAP is more generic and allows access to all kind of Directories; X.500 is limited to certificate chains

C.
LDAP is a lighter version of X.500 as it uses UDP instead of TCP

D.
LDAP is a lighter version of X.500 as it doesn’t require a full OSI stack implementation

E.
LDAP is cross-platform while X.500 runs only on legacy systems