Not trying to get into the age old debate of which is better. From mechanical perspective, all landing gears are just long cantilevered beams. With a greater instability, one would expect tail draggers to not be preferred for a rough ground surface.
To expand/clarify upon the other answer, bush planes and the like use tail wheels because imagine if when landing on a rough runway, your little tiny nose wheel hit a rock. That could be catastrophic. However, on a taildragger, the big main landing gear can go over some obstacles, and the little wheel on the back won't pose a big problem with small rocks and such. The little wheel will just bump over the obstacle, instead of possibly skidding on it and throwing the whole plane off. Effectively, the plane is dragging the tail wheel over the obstacle vs. pushing the tail wheel into the obstacle. So, that's why I think that tail draggers are preferred for a rough surface.
Feel free to correct me if I'm wrong... I've never really flown a plane in my life. I'm just an enthusiast.
Let's assume that the main wheels are more sturdy than nose or tail wheels.
When the plane encounters an obstacle on the runway, the inertia of the plane will push the nose down and lift the tail up.
- the force on a nose wheel will increase
- the force on a tail wheel will decrease
Additionally the tail wheel is further away from the COG than the nose wheel, resulting in less force needed to lift the tail compared to the force needed to lift the nose.