1. The game's difficulty, environmental (such as hallways) is balanced around it, changing that now is just plain dumb tbh. It would delay the development by a long time.
2. Even having 4 characters gets crowded in many places, when you need to jumo over your characters in narrow areas to get to the enemy. 6 characters would eb nightmare.
3. Cycling through characters in combat takes even longer, beng detrimental to game's combat speed whihc is already on the slow side. Especially when you have one character go alone ins neak mode, it's really annoying already to press end turn to skip the turn on the remaining three every turn, Doing it five times would be plain hell.
4. As stated before, the need to specific roles are much less needed in D&D so you can play without a healer etc (I do).
5. Replayability increases by a lot because even after you finish the game there will be a couple more new fresh companions to play with ij your next setup, with their unique storylines to explore.
6. It's more challening, which means more fun.
I used to miss 6 characters as well, but now because the game's so fun and enjouable even after 6+ playthroughs, I prefer 4 man. I will replay this game many times, no need to have all companions with your the first time, and it adds more variety.
I am sure a mod with add 6 man party for you to enjoy anyway.