Back to armchair tneories about Deadfire sales, I really think that the Isometric text heavy RPG shoots itself in the foot because it looks good and plays the same for decades. Planescape Torment still looks alright and plays well, modded to support widescreen resolutions. Because of the fact that the play experience is static, if there is no potential for a major twist that could be spoiled, you can wait for the games to go on sale cheap and not miss much.
In fact, you get a more polished and often expanded experience for less money. Between Steam sales and the like, games culture pushes people away from a sense of release day urgency and toward a more leisurely games acquisition path. Games that are text heavy need to generate serious buzz or spoiler heavy conversations that people want to participate in without spoiling the twists for themselves. If a game like this generates positive reviews but not a ton of spoiler talk, it needs to be a new property or generate interest through other means in order to move a lot of copies at a high price.
There's also the fact that Pillars was a kind of divisive game, a bunch of people bounced right off of the game after 10 or fewer hours play. The fact is that because of the epic length and start of a series plans for Pillars it didn't have the best onboarding. People often puttered around Gilded Vale and got bored before even making it to Defiance Bay. In addition to the normal fatigue, NPCs that served as massive text dumps without any real connection to the lore or backstory introduced a new twist where nothing tells you directly "These flashbacks won't tie into any plot, any world building you need to know, any quest or anything important" and a lot of people seemed to read more of them than they probably should have. By exhausting themselves reading these self contained stories that don't matter, they had less energy to expend reading things that did matter.
I put my nephew in front of Planescape Torment's into and he skipped all the text and said it was boring. He was 17 at the time, just a few years ago. I told him that ofc it was he just skipped past the first chapter of a book and said it was boring. Pillars 1 added a new wrinkle, interspersed with the interesting stuff there were weird asides. Sure, the text color and name color is a dead giveaway that they're different but because the players are often skimming instead of really reading everything like they should a lot of people didn't see that they should basically pretend those characters don't exist. If they hadn't come into the game until Defiance Bay it might have kept more people interested longer.
I can't really definitively say which of these points even mattered the most to the people I do know who bounced right off of Pillars 1, Deadfire doesn't have any of those problems, but at the same time people who see 9/10 on Pillars and bounced right off aren't going to pay full price or 66% or whatever for Deadfire. People can't really tell you in a word why they didn't get into a game. In order to avoid conversations about it they're more likely to give a blithe "it was alright" and not say "I played for four hours hadn't gotten very far and never fired it up again, getting further and further away from it I felt like I'd need to restart and waste another few hours getting back to where I bounced off so I never bothered to fire it up again."
Pillars initial success may have actually worked against it, taking people who were of middling interest and convincing them that Pillars was too old school or too *whatever* for them. For them, they see all the rave reviews and think "It's not them, it's me" and don't see across the board improvements in Deadfire as a reason to pick it up. It's like e.g. Sushi, they think it wasn't for them and the people who love it are just loud about it. They don't interject why they didn't care for it, they just don't spend money on it.