How come I have prostate cancer, but nobody else in the family has it?

Family history helps. Because some cases are "genetically linked" and you forecast based on how well or poorly they did. It is common to have no familial link, and rapidity of rise of psa helps select risks, as well as height of psa, & gleason score. No single psa ever tells a story; you need a collection over time. Many people have prostate cancer and don't know it, and don't need to know it.
Not inherited. Most prostate cancers are not inherited. Some families do exist that virtually all brothers and their father, uncles had prostate cancer but this is actually not common. Even in these families going back before them there was probably no prostate cancer, meaning it has to start somewhere if it is going to be inherited. We do tell all relatives of patients who have prostate cancer to be checked.