In Gulf countries why the job preference are given in the basis of nationality

Hello , from my experience all jobs has different requirements that need various skills , also according to the nature and work culture .
In Gulf countries , the job preferences are given in the basis nationality because of the multicultural work environment .
Based on nationality ( culture wise ) , education and experience its all together

Well the decision may not be based completely on the basis of nationality.
It may be considered if it adds any value for the job role.
Such considerations may not only be in Gulf but would be seen in other places too depending upon the requirement.
However when one walks out of her/his homeland for opportunities has to face greater challenges which must be accepted and enjoyed.
Cheers !