It's the storage requirements and coming from IT I can understand them. But I don't agree with them.
An item stored on your retainer will require something like Item <-> Retainer relation if they have the Retainer <-> Character relation done already - but this is simplistic and probably not very efficient(slow). But this needs:
a) storage - for when not in use and in use
b) memory - for when in use
c) cpu time - for when in use
Now for a single item+retainer+character this isn't much. Let's say the retainer id is a long and the item id is a long and the character id is a long... A long is 4 bytes... So that's already 12 bytes used up for just for basic id's.
Then you have the relation that also uses atleast 4 bytes on each side. So 8 bytes as well. 20 bytes so far. And that's for a single item on a single character or a single retainer.
You already have 175 inventory slots - so 20 bytes * 175 = 3500 bytes for your inventory alone. Add to that the armoury chest - what is it 25 slots per page... so 6500 bytes. So a retanire needs 3500 bytes of raw storage. There is always overhead that you don't see and that needs to be taken into account for such things. It might not be much but it's still there. And with scale it has a negative effect on the performance and the storage.
So for a single character with 2 retainers for ONLY inventory and armory chest storage you need...
17000 bytes or 17 kB. Now multiply that with number of players... let's say 2 milion... that's 32GB(and I would guestimate that the actual number with all the overhead and stuff is probably atleast 5 times larger) just needed in storage. JUST for inventory and armoury for each character with 2 retainers.
Now you add the overhead of all the transactions that need to be logged, backed up, add stack data in there as well and so on and the overhead just goes up higher and higher.
Also cpu, storage and memory needed for good performance is not cheap for this kinds of environments.
So as said I can understand them but as this is a pay to play game I disagree with them charging more for this and they will not see any extra money from me for this even though it would help me immensly(so no not white knighting).
Storing items in armorie though is a bit cheaper since only items in there actually require storage space. There is also no item metadata to store for that - despirit bonds, 100% repaired so as someone said you are in essence junking that item. But can get it back for free.
The above is a very simplistic calculation and omits a lot of things - stacks, metadata(durability, spiritbond, items on sale and for how much, retainer gear).