It's never a computational issue, as much as an attempt from the developer to keep the experience "focused" and have full control of what the UI will need to manage without things getting too messy. Still, we aren't exactly begging to turn this into a RTS where you manage entire legions, just to be a little less restrictive with the default or even just to offer an option.
The UI is also already more or less suited for it, bar a couple of possible tweaks that would help and that frankly would be direly needed even if the limit remains four party members.
Originally Posted by etonbears
I suppose it might have some unwanted effect in MP, but I find it difficult to imagine what that might be; you would just have more players levelling more slowly.
...Or some players controlling an extra character as a follower (which is even simpler to manage). Not to mention that they could still, you know, play just with four if they really want to. Just because we want the option to have more NPCs in party for the single player it doesn't mean anyone has to do it. Baldur's Gate 1 and 2 and Pathfinder Kingmaker are games where the default party is six members and yet they have an active portion of the user base extremely dedicated to coming up with builds and strategies to complete "solo playthroughs". Nothing would prevent the same thing from happening here.
Actually larion stated the ui was setup so that it was possible to be able to have a party of 6 characters but it was not configured to allow it at this time you can actually get this situation during the tutorial if you play with 4 player characters.