Why do some airlines not use containers for luggage?
In addition to the reasons given by @aeroalias, cargo containers also waste space. 

From the A320/321 Operator's Manual:


The A320 can hold up to 7 containers. The A319/320/321 do not use the standard LD3 containers. They use the slightly smaller AKH container.


Each of these has a volume of 3.7 m3, so seven of them totals 25.9 m3. When bulk loaded the A320 has 37.28 m3 of cargo space. That's almost 44% more room.