Why are certain men ineligible for brachytherapy?

Depends: Depending on the extent and severity of disease, sometimes there are other more appropriate treatments.

Disease or anatomy: Low dose rate brachytherapy (i-125 or pd-103)) for prostate cancer is a potential treatment in lower risk or some intermediate risk cancers, or in combination with external beam radiation for higher risk disease. High dose rate brachy is usually reserved for high risk disease, but is far less commonly performed. There are anatomical criteria (size of prostate, pubic arch shape) to be met as well.