Why Formula of calcium carbide is CaC2?

The oxidation value (number) of C atom in carbide (not corbide) is -1, and of Ca always +2. This makes one Ca reacting with two C atoms in forming CaC2.

Calcium Carbide is a common but improper name for CaC2.

It's proper name is Calcium Acetylide.

In Calcium Acetylide there are three covalent bonds joining the Carbon atoms, and

the forth bond of each Carbon go to Calcium forming an ionic bond.

Crystal structure :

The common crystalline form at room temperature is tetragonal, a distorted rock salt structure with the C22− units lying parallel.

(Cf. 'Related links' for ionic picture)