Why are some bodily fluids more of an infection risk than others?
This is just about where the pathogens can be found that are dangerous to people.  

Vomit is highly acidic and less accommodating to microbe growth. Similarly saliva has many immune components in it as well as digestive enzymes that keep most microorganisms down. 

Urine and CSF are actually quite sterile as they come from environments that are highly filtered - the kidney is an osmotic processor that essentially is a molecular filter and does not allow cells to pass, the spine is highly insulated from the blood and other direct exposure to microorganisms. 

Compare that with the 'dangerous' list and you have organs that are open to human pathogens.  Venerial disease like HPV is so common that what - about 1 in 5 people under a certain age carry it.  That is a pretty high expectation of a biohazard.  most infections and viruses are blood bourne - influenza, cold, as well as any bacterial infections. 

Feces is always a dangerous thing to handle as the digestive tract is rich in nutrients and essentially directly open to external bacteria and fungi.  (and its not acidified like the stomach).  Also parasites like tape worms and other multicelled animals! yum!

Diarrhea is often caused by an infection of some sort, so its just more likely a hazard, but feces is always a place where you might find a pathogen. 

This is not to say that the 'safe' list is totally safe.  Its just less likely to bear disease causing agents.