I'm fairly sure these guys were Ultramarines as well, but they had decals instead of the molded on chapter iconography. Honestly, I don't see any faction OTHER than Space Marines of some flavour representing the Imperium in the starter boxes. Obvious cash-cow status aside, they're a VERY easy army to learn the basics of and have an enormous range available to expand into (thus perpetuating their popularity and milking that damn cow!). Hell I did it back in 5th and wound up with a fairly large Black Templar force.
They're also the face of 40k - as important to the imagery of the franchise as the Aquila. Any old scifi setting can have space elves, slick hi-tech aliens, deathless robot armies, human soldiers and whatnot, the Space Marine and Chaos is what set's 40k apart and that is SUPER important for GW, as seen from AoS.
As a long term Chaos player... no idea. They couldn't ignore Chaos altogether coming in to the new edition after the events of the Gathering Storm. The background in the new rulebook has a very heavy Chaos presence. I mean the Great Rift is basically a galaxy-spanning extension of the Eye of Terror, half the Imperium is plunged into darkness and preyed upon by the forces of darkness, Cadia is ruined, Terra was invaded by Daemons, the Thousand Sons have relocated to Prospero following the battle at Fenris and Magnus's defeat at Luna. Shit is going down!
But... why on earth Death Guard vs Ultramarines? Apparently Mortarion has a beef with Guilliman, but it seems Fulgrim and Magnus want a shot at him too (Magnus already being beaten on Luna), and Lorgar's dislike of him goes back to before the Heresy was even an idea! Logically it should have been Black Legion - the most vanilla of Chaos vs the most vanilla of Space Marines. Black Legion already have a MASSIVE presence on the Ultramarines turf and invaded Maccragge itself. It would have given GW the perfect opportunity to replace the horrid existing plastic CSM kits with something more akin to the newer Raptors or Chosen from Dark Vengeance, maybe even release a multi-purpose kit for Chosen/Havocs. Like with Space Marines in general, Black Legion has the most options available and would be a good starter army to expand on.
EDIT: Also Abaddon needs a new model desperately. He's supposed to be THE Big Bad Chaos Warlord, burner of worlds, slaughterer of billions, supreme champion of all the gods. But he is tiny and unimpressive...