I think what it finally boils down to is the loop holes. GW created the Combined Arms detachment to basically be a full army. You're allowed a second detachment, and it doesn't need to be at 2k, so you get another Combined Arms.
The Allied detachment, because it's a 'half' army, is meant so that you can take something completely different... like taking a Guard army with a 'side' of Marines. But because that also meant that people could take just a single troop choice... they closed the loop hole... which also forced supplements to be shut out also.