[chabska]

To deliver very high torque, the clutch plates needs to be pressed very hard together to generate enough friction. This also means that it take a lot of force to pull them apart, if you use a simple lever, as older machines do.

Modern machines may use complex mechanical linkages to make the clutch easy to pull apart but still maintain a firm contact, but that also means higher cost and fragility. Or they use pneumatics or hydraulics to assist, sorta like power steering.

[sandworm101]

That, and design tolerances. A fancy clutch can be light and strong (think ferarri) but farm machines need to work in the dirt/rust and so need larger tolerances. So heavier springs and bigger .... Bigger everything. A slipping clutch in a Ferrari is annoying. A slipping clutch on a tractor means a missed harvest.

[slow_typist]

Plus mechanical release mechanisms of heavier machinery were often designed in a way that the clutch snaps at a certain point (also in order to reduce wear in the clutch).

[sandworm101]

Also because farm machines usually need max torque fast to break loose from static friction. You want a clutch to bite hard when pulling things through mud.