Because it's so amazingly difficult for a straight, non drug using man to contract HIV through normal sex (68 in the UK in 1997), that people just write off the risk. This is a decision they make shortly before contracting Syphilis!
I wonder why HIV rates are so much higher in Africa and STD rates so high in the West. If the level of promiscuity is the same, then why wouldn't Western HIV rates be the same? It's an interesting question.