Giving the option for 6 party members would be way too much work in my opinion. You would need to add additional scaling difficulty, refine every fight in the game, so that it can be played fine with both 3, 4, 5, and 6 party members..
Also having 6 party members would neglect the need to chose your party members, you could just have everything in the party. And all this would only introduce a lot of complications...
I actually feel, the game is much more interesting with a smaller party, and is designed already with 4 party members in mind.
I do not see any actual realistic reason to put in so much work into the game to change this. Apart from: Baldur's Gate 1 and 2 had 6...
And so what?! - It also had a max resolution of 1024 by 768... Bring that back too, and 2D Sprite graphics? Times change, my car was also quite different 23 years ago.
