why is the symbol S for sulphur but NA for sodium and SI for silicon?

Dear student!
Actually, same symbol can not be used for two elements and according to chemical nomenclature, we can take up to second letter or second significant letter only.

Since, S for sulphur and Si for silicon, Se for selenium all exists so sodium has been given a symbol based on its Latin name Natrium (Na). This has been done as Sodium was known with its Latin name too, but the rest elements do not have any other popular name.