Might be due the GPL license of pandoc, this is a copyleft type license, requiring any code that is built with pandoc to be described, made available, and released under the same license.

https://github.com/jgm/pandoc/blob/master/COPYRIGHT

Given that novelWriter itself is GPL licensed, I doubt this is the problem. More likely it was a choice to avoid needing pandoc as a dependency, either explicitly (bundled or integrated in the code somehow) or implicitly (calling to the system's pandoc install), as pandoc is written in Haskell and kind of a PITA to build or integrate in that way.

Since you can already export to a single ODT/PDF/MD, the benefits of additional pandoc integration are probably diminished.