Why the variability in transmission rates of various STDs?

Inspired, as it were, by another thread: Can someone explain why/how the HIV and HCV (hepatitis C) are transmitted by blood-to-blood contact, but the herpes simplex (HS2) virus can be transmitted by any direct contact, such as herpes-infected semen or vaginal fluid contacting a person’s abdomen, leg or arm, according to medical experts.

Adding to that, why don’t the syphillis, gonorrhea, and chlamydia bacteria penetrate as readily as HS2? If it’s strictly a matter of size–the herpes virus being much smaller than these bacteria–wouldn’t we expect HCV, the smallest of all STDs (less than one wavelength of light), to have the highest transmission rate?

Well, HIV doesn’t do very well when it’s exposed to air. My researcher friends call it “A sneaky, cool, virus that’s kind of a wimp outside the body.” Wish I could tell you more, but that’s what I’ve got.