Room 101 is a torture room based on your worst fear. In Winston's case, it's quite easy to find his worst fear (rats) because he is caught by the telescreen admitting to Julia that he hates rats when they see one in their private room. However, I'd assume that for many of the other prisoners, it might be significantly more difficult to discern their worst fear. If no incidents similar to what happened to Winston occur for any of the other prisoners and/or the prisoner isn't caught admitting they're scared of something in front of a telescreen or the Thought Police, how can Room 101 function as intended for them?
The Thought Police know everyone intimately.
This is strongly hinted at all the way back in the very beginning of the novel:
There was of course no way of knowing whether you were being watched at any given moment. How often, or on what system, the Thought Police plugged in on any individual wire was guesswork. It was even conceivable that they watched everybody all the time. But at any rate they could plug in your wire whenever they wanted to. You had to live — did live, from habit that became instinct — in the assumption that every sound you made was overheard, and, except in darkness, every movement scrutinized.
-- Part 1, Chapter 1 (emphasis mine)
We find out towards the end of the novel that they've been watching Winston for years, analysing everything he did and just awaiting the right moment to strike. Why should he have been singled out for special attention? Sure, he'd had rebellious thoughts, but so had everyone who found themselves in the hands of the Ministry of Love. It stands to reason that they've been watching all those people - and probably all people everywhere, just as the above quote suggests - just as closely as they watched Winston. Closely enough to know everyone's habits, everyone's secrets. By watching people that closely, you find out everything about them, which naturally includes their worst fears.
He knew now that for seven years the Thought police had watched him like a beetle under a magnifying glass. There was no physical act, no word spoken aloud, that they had not noticed, no train of thought that they had not been able to infer.
-- Part 3, Chapter 4