1) It's tradition. Past Baldur's Gates allowed for a party up to 6, as does Dungeons and Dragons Online. Fewer party members allowed seems like an artificial reduction designed (I'm guessing) to improve the speed at which NPC actions are calculated, or reducing the amount of calculations overall. I don't know about anyone else, but combat often hangs on my (fairly new) system as the software calculates the next npc actions. Maybe the whole thing just crashes and burns with 6 (and their summoned elementals, etc.)
2) Each character has a side-story. Almost like an ME 2 Loyalty Mission. With twice as many companions as one has spaces in the party, the constant necessity of balancing the party for combat and balancing it for content becomes a little frustrating. With some quests, I find myself having to leave the FOB and return with different party members, and all this juggling of people starts to seem more like a staffing problem than an RPG. It would be really nice to fight Wyrm's Rock with Wyll and Karlach in my party, but as it stands with the Tavs I've played to date, that makes for a poor party.
3) So much of the fun character development is in the incidentals. The way Wyll makes random comments about Baldur's Gate while wandering around the city, or Karlach randomly runs into an old friend. Restricting the party to four members is asking players to miss out on too much of that. In which case, why spend all that amount of time writing and producing it? Sure, re-play factor and all. I'll replay BG3 to test out various builds and explore various decisions, but probably not to catch the little moments that really add to the game's RP appeal. The whole point is interacting with the story, much of which is driven by the characters. Why write a book one has to read five times to get to all the plot (especially when you've left out the ending)?
4) I just find the four-person party reinforces the "Tank, Damage Dealer, Healer, Thief" dynamic that reduces a complex set of explorable options to a well-known and predictable formula. Multi-classing helps with some, but not all, of this. I, for one, don't like Astarion. (OT: I think he's an offensively reductionist stereotype I've seen a thousand times before. Dorian from DAI. Frankenfurter from RHPS. in fact, I thought it was Tim Curry's voice at first), so I end up investing in rogue levels and him behind. I could drive a stake through his heart, I suppose, but it's unclear to me how that would affect Act 3. And since Act 3 is mostly at level cap, it's the story and the equipment (mostly the story) that drives everything at that point. (As an aside, isn't it strange that you can't multiclass in Explorer, but you can switch to Balanced, level up, multi-class, and then switch right back to Explorer if you want?) When there are only four people in a party, and you can't stand one of them, it's a lot more impactful than if you had four other companions you did like, rather than two.
5) Halsin's a dirty, dirty boy. Don't make me leave him behind.
"Often forcing his victims to eat their own lips, he was caught and imprisoned for tax evasion." -Yellowbeard.
