An RJ-45 port is a pretty sizable port, not only W & H but it extends pretty deep. The connector takes up alot of internal space before you even deal with the electronics. The D800 is a smaller body than a D4.
Also, practically all prosumers know how to use USB and most likely already have USB ports on their computers they can plug the camera into. I can hazard a guess that not all prosumers know how to deal with RJ-45 ethernet connections, or even have an available free RJ-45 port on their ethernet switch/DSL router/etc. Unlike the casual use model of USB, devices that are connected via RJ-45 ethernet ports tend to be things you want to keep plugged in all the time, so you aren't going to be unplugging something to plug your D800 in.
Saying all that have they ever developed a "mini RJ-45" port that will take a dongle to convert it to a female RJ-45? That would be the ticket to reducing space use.