I am wondering why people discriminate while hiring for a post while we know the principle it is not fair to discriminate?

If you are in this part of the world, then forget an answer to your question - Since it is not consider as discrimination to mention in ad for a vacant post, that "So and so Nationality to apply" or " Only so and so nationalities will be consider" But if one in the west, that is100% discrimination. 
As for this part of the world is considered as to find a person who can fit in an organization culture.
All The Best Hassan