Man, I was expecting the Saturnine box to disappear immediately like so many sets do, especially with how hyped people seemed about it!
It was honestly more or less a "perfect storm" scenario. Whatever hype there was for the box set was kind of hollow, especially once you start looking at everything else GW was trying to push for this coming edition and how it was handled.
A good number of factors went into the box release being as bad as they were, and while each one is by no means the smoking gun that led to the flop, taken together they made it inevitable.
I'll just be listing off three that immediately come to mind.
1. GW didn't, or couldn't be bothered to, know who their target audience for this game is, let alone who the box is meant for. Having looked over the content of the Saturnine kit, I can assuredly say that, for a box that's meant to be the next starter set for the game, the contents are not suited for beginners outside of the core rulebook. Between the aggressive pricing and the overrepresentation of units that are exceptionally niche, the only ones who were ever going to honestly buy this box were veteran players with money to spend
and an interest in collecting at least some of the models inside. Little Timmy just getting into the hobby would certainly not be able to afford it, and even if his parents were familiar with the hobby and recognized his interest in Warhammer, they would likely get sticker shock and look for a far less expensive entry for him instead. Hell, this is a box set that costs more than some gaming consoles for what you're getting, so I'm not surprised people are balking at the price alone.
2. GW's failure to read the room. Because GW doesn't understand or care who their core audience for Horus Heresy is, they fundamentally do not understand why the Age of Darkness box set performed so well on launch (hint: like Prospero and Calth before it, it was a good way to get bulk space marines for a game that's predominantly space marines fighting other space marines), and Saturnine was their attempt at capturing lightning in a bottle a second time without having learned any of the lessons from their previous success. I would surmise that they started work on Saturnine and 3.0 almost immediately the moment AoD started printing them money, thinking they could get another hit. Coming back to my first point, GW neither realized nor cared that Horus Heresy players, a good lot of them having migrated from 40k specifically to get away from the edition turnover rate, would rather have a good game that only needs minor patches over the course of a decade than a broken game that they need to relearn every three years.
3. GW's inability or unwillingness to communicate honestly with its customer base. This is easily one of my biggest pet peeves with the company. All throughout the lead-up to 3.0's release, the information that GW passed on via WarCom was not just a drip-feed of nothing substantial on its own yet otherwise propped up as hype, but also a lot of vague statements that cast a lot of doubt as to whether anything that was said in the articles was even true. Between the statement "minor tweaks, not wholesale changes" (almost immediately followed by articles detailing what could best be described as wholesale changes to various elements of the game), and leaks that put pre-existing players on edge over losing units and wargear options (some of which there is still model support for in the kits), many 30k players who could have bought in lost faith in the company and demanded a refund (or simply tuned out until release like I did to wait and see).
I'm sure there's more, but unfortunately it'll likely be a never-ending screed about how GW is tanking its own sales of this product in pursuit of greed, and now trying to offload the financial burden on third party distributors to compensate, so I'll stop there.