The reasoning is simple. At age 73 you don't heal nearly as fast as you did when you were, say, 25. To put it bluntly: there's a chance, probably small, that he wouldn't survive such a wound. Since there are other options available, the doctors probably don't want to take the risk (rightly so, in my oppinion).
Hi, these are questions for your Oncologist. Its possible with a gleason score of 8, which is not a fast growing cancer, they feel the surgury would have more complications.