Possibly something to do with the cast iron axle/gear housing?
I don't have answers to any of that.should not be an issue. Dont they all come with subframe option? I dont know, just asking. Im thinking it has to do with hydraulic flow and e hydro. Anyone know how these backhoes get supplied fluid. Does power beyond require e hydro?
What is different is that Deere chose not to make a BH compatible with the 3D machines. All of the BHs used with the Deere CUTs are mounted via a subframe and Deere decided not to make a subframe that fits the 3D. They also don't have the required hydraulics to run a BH on the 3D.
The 3D series was designed to be kept simple and cheap. If you want the ability to add complexity, you pay for it by buying a higher level machine. As the old saying goes, there ain't no free lunch.