bibotot wrote:+ Not including a clear-cut narrative, protagonists, and antagonists.
+ Not featuring "evil" factions like Chaos, Drukhari, and Tyranids.
Stopped reading here

You clearly missed the point of
40k. There are no antagonists or protagonists, everyone is committing atrocious acts for their own reasons, no one is "good".
40k is being extremely successful in a time where sci-fi classics like Star Trek and Star Wars a struggling to stay relevant, precisely because
40k is what it is. There is no need to cater to a "wide audience" when you are already one of the most successful IPs.
But on a more serious note, these games are made by separate studios and separate publishers. They pay a ton of money to
GW to be allowed to use a clearly defined fraction of their IP and they need to run any story they tell by
GW. Obviously they are not taking financial chances by catering to a niche audience like Drukhari. No offense to the passionate players here on dakka, but on other platforms the drukhari communities tend to be somewhere in size between imperial knights and league of votan.
I personally would absolutely love a game where you play as drukhari archon with a mix of raids, assassinations and intrigue games.
In addition, there are over 40 Warhammer
40k games available on steam right now plus a bunch of mobile games and you merely played three of them. Almost every army in the game has a game dedicated to them. Tyranids naturally make bad protagonists because they are more or less a single entity, the individual organisms have no stories to tell. It's more or less the same for demons, both factions are more or less a natural disaster.
There are quite a few games where you play characters falling to chaos. However, once someone has fallen to chaos, it's super difficult make them relatable anymore, plus there is a lot of horror involved which limits audience.
Traditional antagonist factions like orks, necrons or the inquisition have successful games dedicated to them, so it's not the general problem you make it out to be.