The short answer is “bad luck“.
The fact is that we do not know the exact cause of prostate cancer in most patients .
So, at present, we have to say it’s “bad luck” if one develops prostate cancer.
Many things have be shown to be associated with increased risk of prostate cancer.
Sometimes prostate cancer runs in the family and abnormal genes are passed from one generation to other. So it’s “bad luck” if you have inherited the abnormal genes. One cannot change their genes.
Western men are at increased risk compared to the Asians. Afro-Caribbean men are at much higher risk of developing prostate cancer compared to Caucasians.
Many other factors such as bad diet, obesity, unhealthy life style, smoking, hormones, & inflammation have been suggested as possible risk actors but none of the them have been conclusively proven as the cause of prostate cancer.