LDAP protocol vs other Directory Service Protocol

What are th开发者_运维百科e reason one would use any other protocol for Naming/Directory Services other than LDAP when LDAP is being a common standard?

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There is only 1 alternative to LDAP as far as I learnt (after searching and reading few articles) and that is X.500. Below is the conclusion from one of the comparisions

A popular implementation is to use X.500 as the backbone for the directory and use LDAP as the front-end interface. This allows a better performing Client with a more integrated Server network

Disclaimer: This is based on just 20 mins of research.

Resources (http://nlc-bnc.ca/iso/ill/document/ill_directory/X_500andLDAP.pdf and http://www.isode.com/whitepapers/ic-6033.html)