Beamdog is a small company when compared to Bioware. Give them the money it would take to make such a game and you might see NwN 3 released in 40 years.
Wizards of the Coast own the rights to D&D. Getting their permission to use D&D in a game costs.
Wizards of the Coast are a pita to work with (almost as bad as as Atari were). For example they stopped certain premium modules because they weren't set in the Forgotten Realms.
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There's probably loads of other reasons too.
TR