If I recall correctly, the "Gear Up" category premiered during the 2013 Loveland, CO Nats. It was created to "level the playing field" when comparing planes with gear up (GU) versus gear down (GD). This worked in two ways from a judging perspective. The GU planes had fewer details to judge when compared to a fully plumbed GD build. However, the GD planes were more susceptible to alignment problems that the GU planes did not encounter.
So, by splitting them creates a better “apples-to-apples” comparison. This category has been very popular since inception.
HTH – PatV.