I REALLY want to yell at Bioware for this, however, I'm not 100% sure this is their fault.
Look at inventory in other RPGs, and even DA:O. If you carry too many things, the game freezes and lags. I had my storage full of all unique items at camp in DA:O and my game would constantly crash or the save file would corrupt. Skyrim has the same issue. Amalur did, too. In fact, pretty much every RPG I can think of that allows near you to just store tons of things seems to have problems related to the storage of all that information being saved/loaded constantly, except MMOs.
It may just be that, especially with working with a brand new engine, it just wasn't possible. A gigantic castle full of things with no place to store excess seems like a no-brainer, I have to believe it just wasn't a possible/practical option without having to deal with excessive lagging, crashing and/or bugs.