Why is the SI unit of electric flux volt-meter and not something else?

The SI unit of electric flux is volt-meter (V·m) because electric flux is defined as the product of the electric field (E) and the area (A) through which the field lines pass. The electric field (E) is measured in volts per meter (V/m), and the area (A) is measured in square meters (m²). When you multiply these units together, you get:

E × A = (V/m) × m² = V·m

This is why the SI unit of electric flux is volt-meter (V·m). It is a direct result of how electric flux is mathematically defined.