40k is no longer just a tabletop game, its a sci-fi intellectual property that could generate massive amounts of revenue through other forms of media if handled correctly, greatly outperforming the tabletop segment of the company in time (but were still a while away from that). Seeing this only a tabletop game is far too narrow of a perspective.
This show may be the biggest and best way to introduce others to the 40k IP, and like it or not, the venerable mk7 power armoured bolter wielding marine is still the main thing that outside people think of when they hear 40k, and this is mainly dude to the fact that GW has done a great job of cultivating that image though years of video games, books, comics, ect ect. The familiarity with that image to non 40k players is something very valuable to GW.
This is also the main reason why I don't think that regular marines are ever going to be phased out and fully replaced by primaris. This goes beyond the basic game now. Primaris were introduced under the perspective that many SM players, who already bought all their SM and have no intention to buy more, will buy a new SM army because of the aesthetic change, and for the most part it worked! But issues with truescales aestetics and not wanting to buy any more marines aren't something that exists in video games for example, they're issues within the tabletop game itself, so there really isn't an incentive to introduce primaris right here an now.
This also is the case for most third party game developers as well. For example, the new BFG Armada 2 has a ton of new lore in it; Trazyn helping Cawl, Gulliman coming back, Cadia dying, Ynnari and Yvraine, so GW clearly has no problem introducing new lore into their latest media properties, and yet not a single mention of primaris. Really the only game that I can think of that had primaris in it was in DoW 3, and there it was just a cosmetic skin for a Chaplain. I feel this is GW being careful with them for now, and in time they'll be slowly introduced.
I think this show has some incredible potential if handled well, and given the time to set up the universe, Baal, the Blood Angels, the characters, and what the situation is, I can see it being a fantastic way to introduce primaris in time to a wider audience. Sweet Sanguinius, to actually see the Devastation of Baal would be incredible!
TLDR; I think the reason why they're using regular marines and not primaris is really simple, familiarity to those people who know of the IP but don't play 40k.
Edited by m0nolith, 09 May 2019 - 11:51 PM.