Maybe my subjective take of sudden is different, but is it sudden? (aka I progressively succumb to madness over a title)

There've been many fantastic roguelike deckbuilders out since 2020, a little after Slay teh Spire's official release date. It feels more like people have became aware of how fun the subgenre is after the hype Baltaro generated on streaming platforms. If anything is sudden, it's the second-wind of attention we're getting thanks to the above-mentioned game.

I know I'm continuing to split hairs over nothing down here, but 861 games is a little misleading once you get to the end: "Surprisingly, deckbuilders are still an underserved market"

You never know when you’ve reached the peak of a trend, but deckbuilders seem like they’re not quite there yet. Games-Stats tracks 527 roguelike deckbuilders, and Dev_Hell’s Westendorp suggests their higher-than-average revenues, wider revenue spread, and demand make them “relatively underserved as a market.”

So, there's not 861 games, but 527 games?

If you investigate why there's a large gap in reported game listings, it's because Steam is including packs like [Slay the Spire x Backpack Hero] and DLC where Game-Stats is tracking the individual games (i.e, bloatless). This ties back to the title - ultimately we're not trying to answer the literal question, "Why are there 861 roguelike deckbuilders on Steam", because OP never answers that question. Instead, we are answering an alternative interpretation: "Why are there so many roguelike games appearing on Steam in a short amount of time?" The answer, may shock you:

While I've taken those answers from the article, I find it further interesting that they conclude a different question all-together: "Why are roguelike deckbuilders taking off?"

Buh, I've lost it. Ultimately I really liked the core article and their enthusiasm, but I've driven myself to madness here.