Inventory space does NOT equal retainer space. The former is readily available at any point in the world and thus is more difficult for them to store and retrieve constantly. Yoshi-P has repeatedly says they cannot expand inventory or armoury space for these reasons.

There is obviously no shortage of space for RETAINERs because you can BUY EXTRAS.

Inventory spare cannot be expanded even if you throw money at S-E.
Retainer space can be expanded as long as you throw money at S-E.

Big difference!!!

I was honestly surprised there was no free retainer with HW given the sheer number of items added, not to mention the new classes. I got over my disappointment and purchased two added retainers for my account, but I'm also on the 1 character "Entry" plan with no need for additional characters, so I didn't feel as bad paying for the added retainers. But even with four I'm constantly debating on what I'm going to throw out next.