Just because other, lesser game on the downward path of absolute DECLINE decided that as a console player you were too stupid to manage six it doesn't mean its should be embraced as a standard in a PC game.
Console == 4 pads == 4 characters I guess. Or 4 buttons on the cross if you play solo.
Make it work differently for the PC version? Aaaaah....now you talking money.
Would love Larian to make it 6 characters and balance the game properly for hard mode with 6 characters. Ngl. But well, if they don't then they don't. The amount of characters kinda limits possibilities but in the current build 6 characters would be straight OP so there's that.
Some people have pointed out that when they mod in 6 character party, the game actually feels more balanced and satisfying. Also I seriously doubt the limitation is because of D-Pad, typically a game like this uses a more cycling form of selection on consoles, or uses a pop up menu. And legitimately, a console can handle six characters at once easily. Icewind Dale controls fine and so does Wasteland 2 and 3. DOS2 I'd practically have 8 characters at all times cause I love Summoning, and the turn based game could handle it easily and fairly intuitively (minus the weird chain/unchain shenanigans outside of combat to many thats an issue with any amount of characters past 1).