Lots of things but let's focus on just two.
First you're looking at it wrong: it doesn't really matter how busy the calendar is. I mean just because people say that it does matter how many games or creative products or whatever are released, and just because some of those people are experienced execs, doesn't mean that it's true.
People who buy games aren't thinking, man I don't have time for games. Okay? So you see how the first thing to understand is: this isn't for you. Someone who has to make choices about how to spend their time isn't buying many games at all.
Before Silksong, they validated marketability & audience many times. Hollow Knight was in Ludum Dare, Newgrounds, festival & conference circuit, Kickstarter, multiple stores & formats.
Why was it standing out then? To me, the coherence of the art, and a game format that aligns with the indie art production and audience validation well.
Consider that larger games are arted by art directors and teams of contractors. If your whole team is excellent and you spend a lot of money rejecting art, okay, you will wind up with something coherent. You can also have 1 person make all the art like Hollow Knight & Silksong.
Indie games - and these guys are actually independently published and actually just 3 people and a handful of contractors - have this superpower. Any 1 person can make all the art for their indie game, and some have found great success doing this, like Stardew Valley. Some people give up a year in, or ten years in, or whatever.
So if you have little trickles of validation along the way, then you actually finish your game, which is maybe why this all worked.
And then this game is a sequence of levels, you can finish all the art for one screen and then move onto the next screen. You can finish a walk cycle going forward, and record the character moving forward in a single background, and it looks like a finished game. A 3D Hero Shooter requires a huge amount of work on level and character art before a preview is fully arted.