Why is electric charge a scalar quantity?
Nothing is a vector until defined with a direction.
Electric charge is a scalar quantity because charge never graduated into the level of vectors or tensors that need both magnitude and direction.
Electric charge is an elementary quantity born of elements and ions. One of its notable features is that by the time you point it out, it is already somewhere else. But we do know that electric charge can attain a magnitude of force under favorable conditions to become available as power we can use.
We can start by considering atomic charges, which are related mostly to the haphazard buzzing of electrons orbiting and spinning around a nucleus. When these paths were first described, they were neat concentric circles around a central mass. Then the paths became elliptical as depicted in so many illustrations. Today, electron paths are no longer described as paths, but are now called clouds of electrons.
Comparing electron motion to that of an elementary school child, we would see a tiny bundle of energy, bouncing off everything in a totally random trajectory. One of its notable features is that by the time you point it out, it is already somewhere else. There is certainly no definable direction (vector) that can be attributed here.
There are exceptions to the normal movement of electric charge, such as when the elementary students are arranged in a line to go into class or board the school bus. This compares to an electric field applied to the electric charges causing them to line up in a comprehensive order as a result of the external influence.
When the students are on the bus, or sitting in the classroom, they are also temporarily constrained similar to electric charges running through wires or integrated circuits.
In the first case there is a dominant external influence and in the second a physical constraint controlling the motion, but both are short lived when compared to the overall motion of the subjects. Again, no vector can be associated with the motion.