the STN is deep within the brain, and you are irradiating a large portion of the brain just getting to the STN. ALso many top surgeons eschew radio surgery because of the permanent damage that can be done if one misses by a millimeter or two.
Radio surgery is a good alternative for Essential Tremor, but I would still go with a DBS. Its safer, (its adjustible, repairable, and can be removed.)

Charlie