Larian is hard at work right now making patches to fix post-launch bugs and add in content they didn't manage to finalize before launch.
So, they underestimated the amount of time and effort to get certain parts finished in time for launch day. At least they are working to fix that by actually finishing the cut content and have it released via future patches (as they promised).
They will only consider actual DLC after all the content is in.
At the end of the day, even if you look at how shiny the game is, you shouldn't forget that their whole team is only around 40 people and their budget just a few million.
