Question

Why is the chemical formula for zinc trioxocarbonate ZnCO3 and not Zn (CO3)2

Answer 1

Zinc trioxocarbonate is NOT an IUPAC approved name for ZnCO3.

One approved name is zinc carbonate and you should learn to write that name instead of what you wrote. The reason the formula is ZnCO3?
Zn has a valence of 2+. The carbonate ion is [CO3]^2-. Compounds are neutral so one 2+ from Zn combines with one CO3 with 2- to make the compound ZnCO3 with a neutral charge.