The bone is hard and there's no space to accommodate an expanding tumor.
This does cause a lot of pain for patients with bony mets, and so frequently radiotherapy is directed at the largest metastatic lesions in the bone in an effort to reduce the size of these mets, and thus reduce pain.
In the oncology community this is sometimes rather sardonically referred to as "spot-welding" because of the large amount of radiation directed at a small area.
There is no intent to cure the patient with this procedure, only an intent to reduce pain for a while.