Increasing the default party size is not such a simple matter. I've done one playthrough with a party of 6 on Patch 6, and with this many characters you can steamroll almost every encounter using basic attacks and little else. For a proper party of 6, the entire game would have to be rebalanced, most if not all encounters remade.
Don't get me wrong, I'd like a bigger party as well, if only to see more story, but it's not as simple as "just making it official".
The main point is that it would be an option. Nothing needs to be rebalanced or remade.
Furthermore, to what you say here specifically, for me the combat in this game is my least liked aspect of the game, something I view as parts of the game that one has to tolerate to get all the other parts of the game. So with a bigger party the combat becoming negatively affected is irrelevant to me in the face of all the positive gains of a bigger party in all the other parts of the game: a greater range of race and class combos to play with, more story, more content, more quests, more within-party banter, more party-world interractions, more usage of the equipment and items I find/win, more of everything!! What's not to love there?