With the NX-Oticas and X-Tremes, the OB subs are mating to OB woofers. With the NX-Studio, the woofer is sealed so mating it to a sealed servo sub wouldn't be a mismatch. Also, the Studio Monitor was designed to be able to be placed close to the wall. If The SM is mated to OB subs, this ability is lost.
Do the SMs sound great with OB subs? Sure they do, but they also sound great with sealed servo subs. I have two main systems in my house, the full OB system upstairs and a system with a sealed servo sub in the living room. The main speakers in this room are a pair of N3 transition line towers. I have had the Studio Monitors in this system also and they sound fantastic.
The point being that in the NX series, the Oticas and X-Tremes need the OB subs while the Studio Monitors work well with both and if the reason someone chooses the Monitors is they don't have the room to get OB subs at least 3' off the wall, a sealed servo sub is a perfectly capable and exceptional sounding option.