Same reason that they decide against glamour logs and other form of intense storage options. If they could make X specific item uniquely stored in Y location, why not take X and put it into Player inventory instead?

Ask yourself what would be better, the ability to store only materials in 10 slots or obtaining 10 player inventory slots to use on anything? That's pretty much the give and take with a new "storage" system, they take up the same amount of resources plus having to create a new flag to say only "materials" can be stored here would definitely be excessive work.

I think one middle ground that could be met is the following:

Create a new Housing permit NPC that sells the following:

Beastmen NPC gil items:
Beastmen 1
Beastmen 2
Beastmen 3
etc
etc

Basically the NPC would sell the materials that they sell in their territory, so players wouldn't have to run back and forth and it would encourage them to complete the Beastmen storyline in order to gain this NPC's benefits of selling X beastmen sold items for the X amount you completed.

They've already shown how easy it is to add an NPC that sells a multiple list of item via the Salvage NPC, so all they would need to do is code an NPC that sells X items and base it on Y achievements, since the beastmen achievements are potentially what link the stores to selling more items upon unlocking the new rank, or maybe the rank itself. But SE does seem to use achievements/quest to have the Salvage NPC sell what they do.