We were seeing too much flexibility for transport planes to move all infantry. Historically, mountain units and para units had lighter equipment (pack artillery) and were the ones that were often air transported (not regular divisions with their heavier artillery and other heavier auxilery units). for example, the Germans planned air landed a mountain division in Crete once the paratroopers secured the airfields. . .So, historically, it makes sense to limit this to mountain and para units - and it also limits the flexibility of transport planes.
