My guess is that it’s much harder to develop rendering algorithms (e.g. shaders) for NURBSes. It’s easy and efficient to compute and interpolate surface normals for polygons (the Phong shader is dead simple [0], and thus easy to extend). Basic shading algorithms are much more complicated for a NURBS [1], and thus sufficiently computationally inefficient that you might as well discretize the NURBS to a polygonal mesh (indeed, this is what 3D modeling programs do). At that point, you might as well model the polygonal mesh directly; I don’t think NURBS-based modeling is significantly easier than mesh-based modeling for the 3D artist.
[0] https://cs.nyu.edu/~perlin/courses/fall2005ugrad/phong.html
[1] https://www.dgp.toronto.edu/public_user/lessig/talks/talk_al...