It is just a spelling convention. The sequences “ze” and “zi” are rarely used in modern Spanish. The Old Spanish form of the plural is [vezes/uezes](https://www.staff.ncl.ac.uk/i.e.mackenzie/consr.htm); this developed because [intervocalic c generally had a voiced outcome](https://www.staff.ncl.ac.uk/i.e.mackenzie/consr.htm)