hexane-2 4-dione is right nomenclature but i want to know that why 2,4-diketo hexane is not used ?
The carbonyl groups, in this case keto groups are considered as functional groups and should be denoted by secondary suffix, "one", whenever there are no other functional groups with higher priority than keto group.
If you had a higher functional group on the molecule, so that it would take that functional group name, such as a carboxylic acid, then you could have a diketo notation in the name.
But your highest functional group is a ketone, and the ketone ending is -one.
Since you have two ketone groups, it becomes a -dione.