Partly it's about friction and grip - ploughs are dragged through the ground at depths up to 30cm, turning the soil through 180°. The power required to do that is considerable but most critically the grip the power unit (wheeled or tracked tractor, animals) has controls speed, which depends on the conditions. In addition, as speeds increase the soil will not move smoothly over the mould-board and thus not leave a consistent result ready for the next operation.