Fluff Question: Where did CSM back packs come from?

This is a question that has bugged many a CSM & SM player for ages- Why are there different power packs between CSM and SM in the year 40,000, other than the simple fact it makes the models look different (which, as we all know is the real reason). That isn't good enough for many a fluff driven player, and really goes against the FORGE THE NARRATIVE mentality of GW IMO.

I recently saw this in the preview for the next Horus Heresy book.

This is the first time I had seen a definitive SWITCH in the two styles. Even the model for Erebus is using a traditional style power pack.

His is actually even closer to Mk7 Aquilla Armour (40k Space Marine) packs then anything else, which is odd given that doesn't exist and judging by his knee pads Erebus is wearing Mk IV Armour which pack looks like this

The old school plated pack look from way back in the day combined with funky conical exhausts.

Doing some research I found this gem on the "Anvilus" pattern pack, which is in the Horus Heresy book BETRAYAL.

For those who don't want to read the caption it says:

Death Guard Legionary Caipha Morarg (Pict-capture Isstvan III Atrocity)

Morarg was a Legionary assigned to the 24th Breacher Squad of the 2nd Great Company of the Death Guard Legion, and fought with the Traitors on Isstvan III. Note: "Iron" pattern power armour utilizing a prototype Anvilus backpack unit - this variant has enhance stabilizing thruster vents to aide in void operations, but inferior rad-shielding

So that is what the shape is for? Really? Thrust in space. If that is the case then, why don't Loyalists have this at all? Or any Space Marine chapters 10,000 years down the road. Isstvan III is too early in the war for it to be the sole provenance on the Dark Mechanicum. Additionally, it is not like it is an Iron Warrior development as it is on Death Guard and Word Bearers early on, but not them.

If it truly was for void operations, why and how did it 10,000 years later get spread to every single traitor marine. And furthermore, why don't void based chapters, like the Carcharodons, use it regularly. Why is it not seen as specialist equipment utilized by Death Watch or Grey Knights. The only loyal Marines I have ever seen with it are the Space Wolves 13th Company, who have been lost in the Eye of Terror since the Heresy and plunder it from CSM they fight.

And only some of them have it at that.

To me, the reasoning and description of the Anvilus was just an easy excuse and/or was not caught by an editor as a MAJOR PIECE OF FLUFF. While it may not seem that big, it covers in a single sentence with a broad stroke the largest fundamental difference in appearance between CSM and SM.

I for one don't like this description, at least not as presented in Betrayal. Had it come later in the war and produced by the Dark Mechanicum, then it would be slightly more plausible, but still weak IMO.

The problem is though, visually this is probably the best reason given the design of the pack. It tactically has more disadvantages, especially as it grows in size (like the Word Bearer one above)- bulkier, larger silhouette, greater potential for critical damage (shearing off one of the vents).

The only other real explanation is the warp over thousands of years. The problem here is some CSM aren't trapped in the warp for 10,000 years, just fast forwarded into the 41st millennium. Additionally, not all CSM fully embrace Chaos and the warp to have mutations, and for even those that do, why start with the backpack...

However, who knows, maybe that inferior Rad-shielding is the real reason for all the Chaos mutation and not the blessing of the Dark Gods.

What do you think? Is the explanation in the FW HH book good enough? Is there another possible reason? Or does it really just boil down to model making aesthetics?