Can't say for certain but here are some possibly reason. Since nurbs are made of surfaces butted to gether (hence the term patch modelling) they look great can be an nightmare to animate. Converting to poly or subdiv allows a single shell i.e. on continous surface, which makes animating much easier. Polys are also the most common format for most game engines. As for detail, with work all three geometry types can have high levels of detal.