Chaos God of Old Night
Join Date: Nov 2009
Location: The Eternal Night
In my opinion they've chosen Calth for two reasons.
First; it has the standard Good Guy vs Bad Guy trope, the Ultramarines and the Word Bearers are probably the best examples of Good and Bad among the entire Legiones Astartes, so to appeal to the people who want to split the box and play two armies, they can do so and have one loyalist and one traitor army, which will also appeal to the people who want one of each rather than two Traitor armies fighting each other.
Second; it is recognizable. Calth is one of THE battles of the Horus Heresy along with Signus, Istvaan III & V and Terra. They aren't going to do a Battle of Terra boxset, Signus doesn't have two Marine armies so they won't go that route, Istvaan III only featured Traitor Legions (yes there were loyalists there but that isn't the same as a true loyalist legion) so that wouldn't be their first choice, Istvaan V is a good choice but it a true loss for the Imperium in every situation, so the scenarios would be biased towards the Traitors and there would be little room for story beyond the Traitors win. So Calth, which featured a true loyalist and a true traitor legion, which was hard fought with no clear winner beyond the Ultramarines kicking the Word Bearers off the already raped planet, which has a lot of potential for story as we've seen from BL publishing an entire anthology solely devoted to it, and is one of the battles that automatically comes to mind whenever we think of the Horus Heresy.
It was likely an easy choice on their part.