Because if the material (five sense) reality is 'everything' - the only acknowledged reality; then if they control the material realm (which they potentially can do); then they control us - wholly.
(By contrast, when people acknowledge a non-material realm of reality as real and vital - then no absolute control of the population is possible.)
2. Only Mortality.
When our mortal life is the only recognised life - and when people believe that nothing of our soul survives death; then one who controls our mortal life thereby controls our entire possible reality, inescapably.
(By contrast, when a person knows that something of them survives bodily death, then he always has hope of escape from any earthly mortal situation - such hope makes Men harder to control, therefore it is better for them that Men despair.)
Modern mainstream morality is utilitarian - that is, it aspires to provide the greatest level of gratification (maximum pleasure, minimum pain) for the greatest number or proportion of people.
(Or, more recently - since political correctness, the greatest gratification for the people who most greatly deserve it.)
Gratification concerns emotions, and emotions are, in principle, controllable; by controlling perceptual inputs.
For utilitarianism: Those who control emotions control morality. And can have their concept of The Good open-endedly re-directed, even to the point of inversion.
(By controlling perceptual inputs; e.g. getting everyone hooked on the mass media, by training them to ignore or deny the evidence of their senses and common sense, and putatively by modifying human perceptions via technology. And by manipulating psychological responses; e.g. with training, with drugs, or genetic engineering, and putatively with 'micro-chipping'.)
(By contrast, one who regards morality as objective and God-given, cannot have their morality grossly re-shaped or reversed.)
On the plus side; the above analysis implies that someone who recognises the spiritual realm, who has faith in life beyond biological death and who regards The Good as an aspect of God's creation can never wholly be controlled by any earthly power.