It's a long-standing technical limitation. It's easier for them to have more seperate 80 slot spaces than it is for them to have fewer spaces with more slots in them. And the bigger each individual space is, the longer it takes to load in. They already seperated the always-acessible normal inventory spaces from the wardrobe spaces for loading purposes in order to get your inventory accessible more quickly (as equipped gear is always ready server-side and you don't need to wait for equipped gear to load in before it's useable). If they were able to increase the capacity of those spaces, it would definitely increase loading times.