You're going to have to wait until they have a content drought or a season with too few features ready before they will boost vault space. It's a easy thing for them to hold onto and use as a selling point some time in the future. There is no reason other than this that we don't get unlimited vault space, well unless the dev team is really just that bad at coding the game... though thinking about DCV and the reason given, it might be the reason we have such small gear storage.