For me it's because the back stories are way over the top. I've played tons of RPGs in the last 30+ years, disliked some of my companions, but liked most of them. I find it fun to interact with the current bunch, but it's hard to actually like them. They are are either super outgoing and friendly (Gale, Wyll), or in the arrogant to obnoxious realm (Shadowheart, Astarion, Lae'zel). Because of the game mechanics (when we meet, we are level one or two), Wyll can't back up his bravado (Blade of the Frontier, Living Legend, yeah right), and Gale's backstory just isn't believable.
You can't have Astarion *not* having trying to drink your blood, and to me that's an immediate disqualification. Not to mention that when you meet him, he immediately puts a knife to your throat. Lae'zel treats you as if you are something she scraped off her boots. I can actually relate to Shadowheart, because a lot of what she is putting up seems to be a front.
I just can't see why the main character, e.g. me, would be running with this bunch. The way they are introduced, I wouldn't invite them to join me, with the possible exception of Shadowheart, and her only if I tried to rescue her beforehand on the ship.
It's not that I don't understand that they're on edge and it'll take some time to establish a good rapport. It's that these NPC's in particular
are just not well written. Their backstories are too wild. If I showed up to a gaming session and my co-players had written Wyll, Gale and Astarion I would be dumbfounded. Lae'zel and Shadowheart are better, but they're still unreasonably unfriendly, what with us trying to get out of this thing alive together.