Answer: Mainly due to his command of the security apparatus, the extent to which this apparatus was involved in people's lives and the swift retribution for a slightest hint of a transgression, even unintentional.
People from that time know what it's like to lie awake at night, listening to footsteps on the landing. Are they coming for you this time? No...not this time. Deep breath. Or maybe you will hear it. The banging on your door. The terrified eyes of your wife. Being ripped out of your home and led away. Sleep deprivation. Shouting. Beatings. Woman screaming in the next room, sounding like a trapped animal. They tell you it's your wife. Sign this. You hold out as much as you can. No, you can't. You are not a secret agent, you are just a regular joe. With broken fingers, you sign the piece of paper. It says you conspired with your friend to kill the local secretary of the Communist Party. He will be here next. They will show him the paper. He will be sentenced to death and will die cursing you.
Called into a room and given a paper to read. "The panel has heard your case. 25 years of labour camps". Sign to acknowledge.
Transport to the camp. Overcrowded train car. You are crammed in. You are actually hanging in midair, trapped between bodies. On the second day, people start dying and they take the bodies out so it becomes easier. Arrive in camp. Hard work felling trees in Siberia. Cold. You've never been so cold and so tired. Your overseer is a professional gangster, one of the multitude of criminals mixed in with political prisoners. They see a gold tooth in your mouth. Hold you down and knock it out with a tree branch. The guards pretend like they don't see.
One time (you've been here here for how long...black spots in front of your eyes all the time. You are being worked to death), a guard throws a pack of cigarettes just outside the perimeter and tells you: "it's okay, I permit you to go get them". You take one step outside and he shoots you in the back. He tells his superiors he shot you during an escape attempt and gets extra two weeks leave.
You are buried in a grave with a marker denoting your prisoner number. Your family are told you are doing time without the right to write or receive letters. It's only after Stalin's death that your wife (by now, almost an old woman, bent as if carrying the weight of the world) finds out that you are dead. No details, no knowledge of where you are buried, just that you are dead. There's no compensation . In fact, your son is barred from the university he wanted to go to because his dad is officially listed as the "enemy of the people". He falls into a bad company and becomes a drunk, looked down on and eventually dying of brain embolism at 40.
So yeah, pretty much that's why:)