The two main reasons Deadfire flopped is because,
A) There was little to no marketing/media behind it, or at least, media that was effective. I remember reading that PoE 2 had come out and being confused as fuck, especially since I hadn't even got around to the first game yet. There wasn't nearly as much Kickstarter and youtube hype surrounding the sequel, which obviously means less exposure.
and
B) PoE scored so well and got high reviews all around, despite the fact that it was so bland and mediocre. Pillars of Eternity was super popular, and a bunch of people knew about it. This led to a lot of people new to the genre buying the game and playing it. These people that are new to the CRPG genre play the game for a few hours, but for some reason, they just can't get into it. Normally, this would lead a player to think that said game was bad or missing something, but this is where all the good reviews came in. If all these reviewers and YouTubers are praising the game, they conclude that this is what the best of the genre has to offer. This means that their inability to get into the game is not due to the game being bad or missing something, but it's due to the fact that they're not really into the CRPG genre.
The vast majority of people that play video games are fucking retards, and they have no idea what's good and what isn't. You'll see someone on the Codex who played PoE for hours, perhaps even finished it, and will turn around and say the game is shit, while a "regular gamer" will have barely stomached the game for an hour or two, and turn around say "yeah PoE is a good game". They do this because people are sheep, and will believe whatever the general consensus is all the while, they buy and play games that might be completely contrary to what they say they like/say is good (see all the popamole Codexers that larp liking old games to impress oldfags, all the while they've sunk tens of hours into TOW, FO:NV, and Skryim).
So, when Deadfire came along, all of those people that bought PoE the first time abstained from Deadfire, because they just assumed that the CRPG genre wasn't for them, hence the complete commercial failure that the second game was. D:OS went the other way, the people that got played the first one enjoyed it and got pulled in, and when D:OS 2 got all the Kickstarter and YouTuber hype, that core audience along with all the people new to the game came in, which led to its sweeping success. I haven't played D:OS 2, so I don't know if it was good or not, but if it was, I'm sure the sequel will do anywhere from a little less successful, to even more successful, depending on how people liked it.