Fear of death makes people weak

People are made weak when they fear death. This is one reason why the philosophy of materialism is so widely promoted.

It gives people no hope of a life after death, which makes them easier to control- just raise the fear of death to achieve that.

Another other way to control people is to convince them that only those who obey your commands will go to heaven while everybody else goes to a hell of eternal punishment.

To tell the truth about the afterlife would have the opposite effect and remove people's fear of death or of earthly authority, which would severely weaken the ability of power elites to control the population.