There are ways for Bungie to easily increase vault size without surpassing last gens capabilities. (I've seen a couple of well described posts about them but can't remember all the technical stuff they said) Bungie is just too lazy to use them. Not to mention its just a bit more memory while you're at the vault. If anything they won't do it because they know their servers suck and wouldn't be able to handle it and somehow last gen got blamed.