Several reasons:
Skin to skin: The type of skin/sweat glands "down there" are only present on your armpits and groin. So types of virus' that infects that particular type of skin does not easily spread to your hands, faces, chest, whatever. You could theoretically transfer it armpit to armpit but I think that happens literally never.
Immune system reasons: Only your eyes and reproductive organs get a free pass from the immune system. This is why people get ebola and its still present in their eyes months later. Generally speaking testis, ovaries and eyes get a free pass from alot of white blood cell intervention... making them viral and bacterial reservoirs
Skin breakage: Gives bacteria and viruses access to the blood stream. This is mostly male to female particularly around the aunt flow time of month. Female to female transmission is pretty low for most STDs and its stupid high for Male to Male because there are virtually no natural defenses "that way" You can even get butt herpes, just saying.
Further skin type: Mucous membranes "down there" are a similar skin type to what lines your mouth actually, and so there is also alot of Oral STD transmission.
So yea, go hop on Tinder and boink in the woods with like 5 different people... see what happens....
You can actually tell people who have had alot of sex partners have bad complexion from the skin viral load, IMO.