Because the high clearance kit removes and replaces the lower rear brackets that hang down from the axle. Those brackets are used for the lower control arm as well as the shock mounting. With a coilover system like EVOLever, the shock mount is no longer needed and the EVO high clearance kit allows the lower control arm to be positioned higher on the axle, thus high clearance.
This is a good explanation. Sorry I must have missed this question being asked.