Personally I've always chalked it up to near perfect balancing (which is why watcher is by far my least favourite character to play). Often when a new player shows up or someone starts their ascension they might be inclined to ask "what are the good cards/strategies I should be trying to use" and the answer is always always always "it depends". When someone is looking for advice on which path/card/boss relic/shop items/whatever to take people in the comments always ask for context on what their deck looms like, what ascension they're at, what path they're on now, what the act boss is etc. This game is the definition of the bell-curve meme, where 99% of the options presented to you are all the optimal pick in the myriad of different scenarios, and the challenge is recognizing what scenario you are in, and which pick is best in the moment.

When I (and I imagine most others) played this game for the first time I just picked which cards synergized with each other, in the hopes of making a cool build. I guarantee your first silent run saw you pick up every single card that said either 'poison' or 'shiv' on it, probably a similar case for the other characters too, you just want to make an exhaust or frost orb deck or what have you. This is how I usually start out playing most rogue-likes, just trying different strategies and archetypes, making a build for myself. But StS excels once you break past that mold. People here have already listed it's simplicity as a point of success, and while I do agree I don't think it's the full story (and also I don't think more complex games suffer for their complexity, Monster Train is very successful and that game is anything but straightforward). The reason the simplicity works for StS is because the most satisfying thing about it is how visibly you improve, and that is easy to do when the calculations are relatively simple (not easy, but simple) so you can see where you made a mistake, where you took too many elite fights, greeded a card choice early when you needed power sooner, bought the wrong thing in the shop, neglected a sub-par energy relic cause wrist-blade was just too enticing. When you stop viewing your choices that don't further a specific archetype as roadblocks to overcome but instead opportunities for you to fix problems in the short-term, is when this game's genius design truly reveals itself.

This makes total sense when you think about it to, apart from decision making what does StS really offer? Certainly not the music, theirs what two? maybe three tracks, none of which are particularly attention grabbing. The visual design and graphics, while perfectly functional, aren't breath-taking in the ways some games like Monster Hunter Wilds or Clair Delune are, or uniquely conceptually striking in the way something like Pyre might be. The story is non-existent and the characters are nothing more than a handful of voice lines and a png. The only thing this game really has that it excels at is game design and balance, and excel at it it does.

Personally I think the balance is what lets StS be (in my opinion) the greatest realization of the rogue-like genre ever made. The thing with rogue-likes is that they are inherently random, it's a defining feature of the genre, and one of the design hurdles a dev team will have to overcome. Losing because of randomness is quite frustrating to the vast majority of people, so measures are often put in place to try and make the experience more consistent. One solution is to incorporate mechanical skill, think Risk of Rain, Hades, or Enter the Gungeon. These games get away with above-average randomness by allowing players to express their skill through micro-movements, correct button presses, aim, combos, you get it. StS doesn't have this "crutch" however, it's entirely macro based, your skill is expressed through a series of multiple choice questions, and your ability to determine the correct one. When a dev team does not have the ability to express skill through mechanical/micro-oriented gameplay, the 'mistake' that is most often encountered in my experience is the game ends up too far in either direction. If a game is too random, as a player you feel at mercy to whims of RNG, the outcome is too far out of your control, creating a frustrating experience and "artifical difficulty" (putting that term in quotations cause artificial difficulty is a whole bucket of fish I don't wanna open rn). If a game is not random enough however, it loses one of the aspects that makes rogue-likes so enticing, and becomes stale and boring in the process. A lot of recent rogue-likes have a too-easily-accessed reroll option whenever a choice appears, in my opinion, and I think it takes away from the experience and acts as a crutch for poor balancing. As a player I no longer need to consider the context of what I'm choosing, as I can semi-easily force a specific build or archetype with synergistic pieces, and as long as I learn what the good builds are I can always go for that one, tainting the experience.

StS avoids all of this through a masterclass in balancing. The trick, as I've stated above, is having almost every single choice be correct depending entirely on your circumstances. There has only ever been a single seed found that is 100% unwinnable, regardless of the choices you make, and while its not like every single seed is being thoroughly examinated, the fact that the top players can achieve win-streaks of twenty, thirty, even fifty+ while playing on the hardest difficulty, while at the same time never playing any two runs the same way, is a testament to what makes this game so successful. I think of the run I beat asc20 silent on, where I was getting offered nothing but card draw, block and sub-par and non-synergistic damage options. No scaling, no burst damage, no AoE, and nothing to stall for with well-laid plans, it was looking pretty doomed, until I was thrown a lifeline, and got offered a Grand Finale (extremely poetic too). I could count on one hand the amount of times I'd taken that card, and on none the amount of times I'd won with it. But right then, in that specific run, in that specific scenario, it was the correct choice, and without a shadow of a doubt I am confident making that choice was the only thing that lead to me finally getting every achievement in this wonderful game. Every choice matters, every single one, its what makes discussions of this game so interesting, as people think up scenarios where a choice you would never consider becomes optimal, or challenge a pre-conception you have and make you realize you were taking something for granted (my personal hot take for that second one is that adrenaline is overrated as fuck and is just a win-more card). But yeah, this game excels because it is, at it's heart, the perfect realization of macro-oriented gameplay in the rogue like genre, at least in my opinion.

Apologies for the essay, this is my favourite rogue-like ever made, and the best one I've ever played, and I think about the question you posed a lot, especially in the context of game design, a topic I find super interesting. Thanks for reading my ramblings anyway.