Why on Earth are people saying that the game would have taken years longer to develop if it had 6 party memebers? That makes no sense. How would increasing the party size from 4 to 6 require years worth of programming? Don't be silly.
4-man party size in D&D video games has always been awful because the game is designed around the assumption that a party will contain at least one of each of the four core archetypes, so when that's already a full party, it leaves very little room for party makeup variety. While you can play through the game without a rogue or warrior or whatever, it's pretty obvious that D&D isn't meant to be played that way. That works alright in tabletop where there isn't the same need for replayability, but in a game that belongs (nominally, at least) to a franchise whose entire essence is the notion of replayability for literal decades, a 4-man party size is an extremely bad design decision. This is not D:OS where every character can learn any skill.