This is probably a design decision by the team, so I may not be a good source to answer this question. However, my understanding is that this may be done from a performance (and necessity) standpoint.
As we are using eager loading in Entity Framework, it means all the related entities will be retrieved when we retrieve a single entity. And with the object graph as complicated as nopCommerce's, it can degrade performance to do this. So for those entity that is less used, we may save some memory by not retrieving them.
Just my assumption though, may not even be a valid reason. :)