The main reason for the limitation on the inventory system was the fact that the game saves character data twice as often as most MMOs do (every 15 seconds instead of every 30). As you increase inventory space you increase the amount of data to be written by a substantial amount and therefore it made it difficult to give us more space. Yoshi-P has said that they're reworking the way the game saves data to make it more efficient, which will allow them the ability to give us more space, but the change required new hardware infrastructure to do so which has been put in place for the 4.0 launch already.