So, why don't we get Daemon Princes of Chaos Undivided?
They supposedly are in the 40k universe: Krieg Acerbus, M'kar, Mortechaan, Kor Megron, etc; but not in the game. So, was there a major change in the background while I was busy? Are there only Be'lakor, Perturabo and Lorgar now?

TBH, I don't think BC's writers know their fluff all too well beyond the narrow scope of their own interest. A lot of the stuff in the tomes is poorly researched, and at times in direct contradiction not only to other publications, but their own product line. The franchise could use a reboot with a writing team that actually likes Nurgle and Khorne.

I don't think FFG is necessarily at fault and to be fair this seems the way that 40k is going in general. The problem seems to be that many of the undivided daemon princes have also fallen out of the current line of 40k. For example the fifth god Malice is now only referenced in a side bar and only as part of an item description and it seems that the Empire itself has reduced the mentioning of unaligned daemons and gods quite a lot. There's probably a few scattered mentions of them in current books but it sounds on the whole like GW are lessening their use as time goes on and making vast retcons.

I would suggest it was the GW who pushed this fluff. Disregard this "no unaligned DP" bullcrap and housrule it in your game.

GW has been trying to do away with Undivided more and more, and I personally find it ridiculous that they aren't instead reinforcing the idea that Unaligned and Undivided are two different things.

But yes, the fact that there are no Unaligned or Undivided Daemon Princes is utter and complete bullcrap.

FFG have to meet with lore experts from GW when they research for a new supplement. So if it's anyone's fault its GW.

Homerules are a good thing, but a mistake is a mistake, and it should be fixed. Let's hope they'll take it into account.
If they read the forums...

I agree with you 100%. Unfortunately FFG seem determined to defend their current stance through re-invented fluff so I think it will have to be home rules for a while yet for our beloved unaligned and undivided daemon princes.

FFG have to defend it though. If the Empire says no then FFG must go with it. They have SOME scope to add things but there will be a limit to how much they can tweak or make stuff up. If they go against the guidelines set by GW then they may find the book rejected or worse, licence revoked.

I agree with you 100%. Unfortunately FFG seem determined to defend their current stance through re-invented fluff so I think it will have to be home rules for a while yet for our beloved unaligned and undivided daemon princes.

To FFG:s defence, whatever deal they have with GW likely stipulates that they aren't allowed to comment on GW interference - which by itself is part of the problem, but that's somewhat beside my point.

Sometimes, the rules for what rules are allowed to be written seem oddly specific and just plain weird, but it's not like FFG can throw GW under the bus and just go "Hey, we wanted to do it, but GW said no."

GW regularly run around like a group of headless monkeys bullfightning elephants in a whole chain of china shops, has an extremely narrow focus in regards to their IP(:s) and a terrible business sense, so these things are pretty much par for the course, unfortunately.

While we may not agree with FFG's way of handling it, it is a valid decision and one that quite possibly fits with the BC setting (Screaming Vortex).

Snippet from the Core Rules Book.

Of all these paths, the path of the unaligned is the one where a Heretic may remain truest to himself. Though he gives each Ruinous Power their due , and may invoke the names of any and all of the Dark Gods, he resists becoming dependant on them. In turn, while he may not receive their favour, his will remains his own

Emphasis mine.