why electric current is a not vector quantity if it has both magnitude and direction

Current is not a vector because despite direction it does not obey the triangle law of vector addtion.

Imagine two current carrying branches meeting at a juntion/node, you do not add currents at junction vectorly. The currents in the two paths add numerically to equal the current coming into the junction I = I1 + I2

Quantities which have direction but do not follow the triangle law of vector addition are Known as Tensor Quantities.

In fact tensors are merely a generalisation of scalars and vectors; a scalar is a zero rank tensor, and a vector is a first rank tensor. Current is a zero rank tensor which means it is a scalar quantity.