Larian has stated that the engine was capable of supporting six character parties before and that the players would be able to mod the game to do so.
The reason for why they don't allow it is because they want the game to have (and thus balance the game for) a four character party. Which in the end is the most acceptable argument, even if it's not what I would like.
So far the only party size mod I can find on nexus is
this which us just a description on how to mod your own save games to allow it. I would prefer it if Larian embraced the wish for it and allowed it as an option in the menu or in a configuration document or something though, rather than having to hack your own saves.
