why sodium hydroxide is not a primary standard?

A primary standard is made by dissolving a known mass of a chemical in distilled water to form a known volume of solution.

Sodium hydroxide absorbs moisture and carbon dioxide in air. After the absorption of moisture and carbon dioxide, the weight of sodium hydroxide is greater than the pure sample. This causes an error.

Primary Standard

because it is hydroscopic

Fundamentally because it does not replicate well on repeated measurements. Its behavior is inadequately constant. We want our standards, our points of reference, to stay the same without regard to conditions. things that do not, are generally not very good reference points.

Primary standards must be available in a high state of purity which sodium hydroxide is but unfortunately it doesn't store well as it picks up moisture from the atmosphere rapidly. The result is that it is impossible to weigh accurately which is THE main requirement for a primary standard.
Less importantly it absorbs CO2 from the atmosphere. In most applications this is of no concern as the sodium carbonate produced will still be titrated by acid.

CO2 from the air will constantly react with it changing the concentration.