Why doesn't Bruxelles-Propreté use containers instead of bags?

We collect in 3 ways: bags, mechanised containers and sorting areas (underground containers). 
Bags are mainly used for houses and small buildings, mechanised containers are mainly used for large companies with sufficient storage space and large buildings, and sorting areas or underground containers are used for new or recently built blocks of flats with more than 150 units.
In addition to these collections, Bruxelles-Propreté manages a network of bottle banks throughout Brussels.