I think something like Dark Souls is so popular not because of one aspect, but because it is more than the sum of it's parts. I think the IE games were like that.
My understanding is that a large part of the CURRENT popularity of the Souls games is their infamous challenge level. The Dark Souls games are still very much known as insanely challening, mechanically demanding games, and all the Souls-Like game are billed as much the same. Completing a Souls game is an achievement by the very nature of the games.
Completing Deadfire, even on TOI POTD, isn't seen as as much of an achievement, because it's a knowledge based challenge, which really isn't regarded as impressively. I'm not honestly sure there can be too much comparison.
Hmmm maybe accoring to unoriginal games journalists. But lets face it Demon's Souls and Dark Souls are a league ahead of 2 and 3. Because the latter have just latched onto the difficulty bollocks, not what actually made the games good. But i digress... what were we talking about?