Why do some guys have prominent veins on the penis?

Cuz it is normal: It is normal to have veins in this part of the body. There will be variation from person to person as to how prominent the veins appear. Look at the forarms of different people, some people have prominent veins and some people have ones hard to find. Generally thin people with a low percentage of body fat are going to have veins that stick out more.

Penile Veins: All men have prominent penile veins. These veins can enlarge and decrease in size normally. With an erection, these veins engorge with blood and this contributes to the erection. In its flaccid stage, the penile veins will be less prominent.