NWN used a modular 3D system that enabled people to create their own dungeons/overland areas with ease. For BG you'd have to get people to hand-draw excellent background maps and then get someone to programme the game into them, which is a lot more work.

As the devs say on Twitter, there's some stuff they can do, but it's limited as BG was never designed to be fully customisable/modular from the ground up, whilst NWN was (and part of the reason why it's a bit bland, as the textures and designs had to be applicable to all manner of fan ideas and modules).