Why is potassium hydrogen phthalate (KHP) ubiquitously synthesized and used as a primary standard, while sodium hydrogen phthalate isn't even available to buy?
For classical analytical problems you have to check at least 100 year old literature. It is easy in Google Scholar. Set the date limits to 1920s or 1950s whatever you wish to try. In older times, the conventional wisdom, which still persists, was that one should use a higher molecular weight standard. The rationale was that the relative error will be small on a balance. KHP has a higher molecular weight than NaHP. This is one advantage. Second, advantage is that it available in extremely high purity. It can be dried as well.
Both KHP and NaHP work perfectly. There is a caveat with NaHP, that it crystallizes as a hemihydrate i.e., for two NaHP units, there is a water molecule. KHP crystallizes as a normal anhydrous salt. Analytical chemists run away from hydrated salts because you would never know what is the hydration level, what was the humidity level at which they were stored. We would never know the "true" molecular weight of the substance which is being weighed out. There are salts such as ferrous ammonium sulfate with water crystallization, but it is very stable with respect to storage. NaHP, who knows how it loses its water of hydration.
Search "Acid potassium and acid sodium phthalates as standards in acidimetry and alkalimetry" on Google Scholar. The reference is Journal of the American Chemical Society 37, 10 (1915): 2352-2359.