I doubt it is really connected to that. It's just easier and faster to do this multiplayer maps with ai + story == new campaign!!11 Especially considering that this was introduced since Relic is adding more than only one playable faction to the games campaigns.
Example: DoW: Winter Assault had a real campaign, which also was pretty cool, imo. But Winter Assault also only got two playable factions (Imperials and Chaos(?)). Then came the next addon, Dark Crusade, which added the Tau and Necrons + what was there already in the main game and to give every faction some missions to play, they "dumbed down" the campaign structure and used this totally generic multiplayer map mission system. Soulstorm did the same... more factions and because players want to play singleplayer campaigns with their loved faction, they generalized the campaign again.
Looking at DoW2 now, it is exactly the same thing. Chaos Rising = one playable faction and a good campaign (so far). With Retribution, the campaign was once again extremely generalized, so you can play it with each of the many factions you can chose from. That's totally quantity instead of quality.
So... yeah. I am very certain they are able to create good and fully fleshed out campaigns. But only for one faction at once, not 5, 6, 7... For so many playable factions, they have to generalize the missions and maps in a campaign.