It's because all of those races have all been playable in 5e, some for many years, people get used to having their favourites. In an actual pnp campaign the DM can make allowances for most things (unless there's a world lore reason, or you have a really restrictive DM, they do exist), but in a video game campaign it's simply not possible to code in specific reactions and allowances for every possible player race, because there are a lot of exotic and monstrous player races. It's too many variations for what is essentially a heavily railroaded campaign.
Larian is clearly taking the path of less effort, by just not coding extensive npc reactions to the more maligned or exotic race options we have available, and at this point, I'll be happy if we actually manage to get all the PHB options by launch, even if no one really reacts to a Drow or Gith character (because then it'll just be exactly like a dozen other dnd crpg games).
What? The drow has arguably the most unique dialogue in the game. What's more, it's literally easy mode in any interaction with the goblins.
