“I don’t understand why it can’t have windows.”
They are a structural weak point. The body is acting as a wing (of sorts) and subject to more strain and is more of a “rivetless” (not exactly correct) construction with much larger pieces you don’t want to compromise by cutting holes in.
And, although this is minor, windows (and rivets) mess up the airflow, similar to why to polish ports in a carb.
We actually had the discussion earlier about the Natilus Horizon (a more extreme blended wing aircraft, but which has similar design features) that is also going into production that similarly has limited windows for these reasons.
If you aren't going to have windows, there's no reason to not go to a full blended wing design.