Some men are not suitable for surgery for all sorts of different reasons; because of the extent of the cancer, or because of previous abdominal surgery / scar tissue, because of heart problems, fear of anaesthetic, living on their own with no support network post-op. Others choose RT or brachytherapy because they are just not prepared to risk being left with permanent incontinence or impotence. Sometimes, the multi-disciplinary team advises that all treatments are suitable but there is a concern about whether surgery could get it all, or conversely, the MDT might advise that no treatment is necessary and close monitoring is the best way forward.
My husband was 50 at diagnosis and if brachy had been available that would have been our first choice, but he turned out not to be suitable. My father-in-law, on the other hand, refused to have surgery or hormone treatment because he wasn't prepared to lose his sex life. He died within 4 years which is a pretty certain way of losing your sex life.