The tents are used for basic training, and for courses of that sort of duration a big room with rows of beds works.
The green sector in St-Jean is essentially a big room with bathrooms and laundry in the middle, and beds around the edges.
For anything past basic it is an issue of retention for courses that are longer in duration, and/or require more academic work/study.
Until the 70's people were accustomed to living with siblings in a shared space. That hasn't been the norm for 50+ years now, as families have shrunk and house sizes have grown.