Good question Mr. Poke,
The reason why your arms and elbows stay dry, and sometimes even ashy, is that they are both in areas with a good amount of ventillation, whereas the scrotum, or ball sack, as we like to say in the medical field, are in a humid, almost tropical environment.
I suggest a splash of talc or baby powder, in the crotch area of your briefs, to help in controlling the marine life that may be populating.