Its the only way you can get ground clearance from the differential, the differential being the lowest point of a tractor, on four wheel drive tractors where they have a differential at the front as well they also have large wheels. lots of ground clearance needed for spanning crops etc so not to damage them.
Ratter is absolutely right of course but it would take a very long armed farmer indeed to reach his thermos and ploughmans lunch from the front mud guarde, the rear mud guarde however is conveniently situated from a seated position.