its been stated that Vault space is a technical limitation
Namely the weapon comparison stat card data doesn't all fit into ram simultaneously when inspecting a proposed enlarged Vault
Here's a hilariously clever solution Bungie
Since the Cursor has a set max movement speed and is not truly random access, we can take advantage of the time delay to load different sections of the Vault into ram cache
Just reorganize the Vault weapon data into a binary space partition tree, then collision check cursor position against it to decide what to load
Easily cut the limit by half even in worst case
-
Or they could make separate vaults? Or separate, pages in the vault that load into RAM, last gen is an excuse to not pay for more server space to save our personal information about the weapons and gear we have because if they doubled our space they would have to double server space costing them more money, and honestly we've already seen Bungie is now all about profit and could give a shit about player wants and needs.