TL;DR version: How tyrants hold power, in general, and how did Stalin?
I mean, for example, Hitler was voted (the "enabling bill") as the absolute ruler of Germany and his will became legally law (let's not debate legitimacy here).
But, as far as I know, the supreme power in soviet empire was never delegated, legally, to Stalin. The supreme governing body was the Politburo. Still, somewhat (how?), Stalin controlled the Politburo.
How could that have been? A motion for his instant dismissal, taken with a simple majority, could have toppled his power instantly. Mussolini had that surprise when he convened a meeting of the Grand Council of Fascism. But, fact is, the GCF was not convened unless Mussolini wanted and his dismissal came after many years when the GCF was simply not convened.
Still, nothing similar happened in Kremlin. The Politburo was convened regularly (or, quite often), a simple majority would have dismiss Stalin immediately, people in the Politburo were quite harsh politicians, so I doubt about moral issues and cowardice, they saw people killed around them, so they must have some hints that theirs own lives are at stake.
A simple vote would have suffice and the occasion presented herself many times.
Was a law? A custom? What prevented such coup against Stalin and what was the legal, or maybe also the psychological base of his power? Why the Politburo simply did not dismiss him but accepted quite easily even killing among its own members? I imagine others too were hungry for power.
Other oligarchies that come into mind (the Khmer Rouge regime, the Argentinian and Greek juntas, Franco regime etc.) were never submitted to such degree to a single person.
On a larger scale, how others dictators keep such a tight grip on power? For example, when they are abroad, any meeting could topple them. Still this rarely, if ever, happens.
Edit: Thanks to Semaphore, let's admit fear as a reason. But, still, the Politburo members were the most powerful people in the empire and their votes counted as equal weight with Stalin's. A simple bill passed against him would have been enough. I think police and army would obey the new master, even if a bit reluctantly. Not much later, Krustchev denounced Stalin and it was not a general uprising against him. So, if it is fear, what made the entire Politburo so fearful so they accepted kills even among themselves? I mean the real reason, they were 10 people in room, during a meeting, no arms, no guns, no monsters inside. They were not handcuffed. Anyone could pass a bill and it would have been over.