Quite simply because that same side overwhelmingly controls the dissemination of information to the public and have the motive, means, and opportunity to make u

Quite simply because that same side overwhelmingly controls the dissemination of information to the public and have the motive, means, and opportunity to make use of that monopoly/power to push their belief system as the "truth". Combined with the average intelligence of the population being pretty low and unable to see through the manipulation.