I do notice that in the transition from 2D-isometric to 3D, almost every game that kept the "party system" transitioned from parties of 6 to parties of 3, or 4.
Probably because, especially initially with early systems and 3D, it wasn't easy to figure out how to get players to simultaneously see and control 6 chars instead of 4 (though the overhead/tactical camera works nicely, when it's there), and I think the other issue was the problem in rendering too many moving entities (party and enemies) onscreen.
Dungeon Siege let you take 6, but 1 was a pack mule, and the other 5 were autobots.
NWN2's OC1 and MotB gave players' party control (unlike NWN1's companions), but limited you to a party size of 4. (BTW, there were console codes to overcome it.)
NWN2: Storm of Zehir (the last 'official' expansion/campaign/addon for NWN2) let you control 6 chars (first "dev-OC" to do so, I think, but also the last released) ... ahhh, if it had been a higher level game, where you could get into the epic levels past 20, I might still be playing it now.