For Versus I think it's because it's the only PVE mode where you can progress your challenges, since the XP changes have made it a lot easier to level up, I would guess that is why they removed it and they'll re-add it but without any XP and/or challenge progress.
I like how TMCC does it where they have challenges specific for Campaign too and some for Firefight, obviously Infinite doesn't have Firefight yet so they can't do that, but I think some Campaign challenges would be nice.
Campaign challenges would be awesome! I love playing the MCC campaigns from time to time and being able to unlock more from the battle passes is like icing on the cake. Especially now with campaign co-op finally added to Infinite, it'll give players more incentive to replay the campaign outside of just achievements.
Speaking of which, one of the new campaign achievements is bugged AF! Where you have to get a kill from every weapon in the opening mission. TrueAchievements is pointing out in their guide that many are having issues getting it to pop.