Why do some airlines not use containers for luggage? Many planes allow the use of ULD containers to store hold luggage: such luggage is placed into the containers, then the containers are stored in the hold of the plane.

There are many advantages to this, as explained here: Why is luggage required to be loaded in containers?

However, some airlines do not use ULD containers even when that is possible. I have in mind Easyjet on their A320-family planes, which don't use containers, at least on some flights, even though the planes definitely can handle them.

What could be the possible reasons for this? Given the high emphasis on saving every single penny they can, I suppose they have tried both approaches and come to the conclusion that not using ULDs is actually somehow cheaper than using them, which seems counter-intuitive:


it should take longer to load and unload luggage one by one than just move containers in
more manipulations increases the risk of damage to the luggage


Possible reasons I can guess at (but not verify) are:


loading the containers is actually not so much faster, with containers possibly getting stuck or some similar problem?
loading containers requires more equipment, and turns out to be more expensive for this reason
it depends on the amount of luggage being carried?


Does anyone have any insights on this?
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.