I'm with the devs on this one. The level of complexity with making a MP/co-op is a considerably large undertaking, and it would take away from the experience of the game itself. I don't mean that in the "the money is going into making the MP aspect and less into fine-tuning just single player."
The way I see it, most RPG's have to have some sort of character creation, some way of putting yourself, or a character you imagine, in game. The problem on the developers part of this, is trying to make your character seem like a believable part of the world. With more CC options, the more work they have to do to accommodate those potential choices you make. The simplified version is whether you make a male or female character - the dev's have to make sure to get those pronouns right
With more players in game, it becomes more difficult for the game to react to each player in a meaningful and satisfying way. Example: in Baldurs gate, if you weren't the Bhaalspawn, you were essentially just a nameless, faceless Mook. And that, to me, feels very immersion breaking. I may not be the "hero," but I was there to help out my friend fight those big fights, and make those big decisions.
And so, in writing a story like that, they would either have to just lump every identifying mark as a "what a great group nameless adventurers!" or make one guy the bees knees and everybody else his designated "gang."
Not to mention all the technical aspects of it that were mentioned.
And to be honest, I'm perfectly fine with the devs decision that a MP/co-op was just not what they wanted to make, and leave it at that. Me and my girlfriend are probably still going to play it side by side and compare experiences, which to me is still a fun group experience.