Because of Working in mud and up and down space if suspension there counter force is more and its can not stay in stable position in that time.

 tractors and Backhoe loaders are supposed to maintain a constant depth in ploughing/digging. For this it requires the movement of the body to be restricted so that we dont disturb the main tractor functions.

If we have a suspended tractor which bounces too much over undulations in a field then we cannot expect the implement to have a constant ploughing depth which is a requirement in agriculture.