why should ac current is a vector quantity but dc current is a scalar quantity

Electricity may be static or dynamic (electric current)
Static electricity is scalar though it can be + or - signs, but nothing other than that.
Coming to current electricity, the dc current is scalar, as it has only one sign and does not get resolved.
Alternating current is vector. The current and voltages are not in the same direction always. Consider a capacitor for example, it may have a voltage of 100 V applied to it and might have current of 0.1 Amperes, but the power is still zero as the phase difference between current and voltage in case of capacitor is 90 degrees. In resistor it is 0 degrees. If you have series connection of capacitor and resistor, the angle between voltage and current will be between zero and 90 degrees.
The current can be resolved in direction of voltage and quadrature to it. Therefore, the alternating current is a vector.

Current and voltage are vector product in a.c. only in wave from . In d.c. current is scaler product because it has only one sign and can not get resolved it.

In the case of AC current or voltage for each instant there is a unique magnitude as well as direction hence AC voltage and current are vector quanties