Most vaccines aren't 100 percent reliable. Either because of the researchers' inability to formulate an effective vaccine (for whatever reasons), and because of how an individual dog's immune system reacts to a particular vaccine. Not all do so robustly enough to develop full immunity. And of course there are the zillions and zillions of pets whose owners don't get them vaccinated on time, strays, etc. And there are a few breeds (Rotties and Dobies are two that I know of) that are more susceptible to parvo than other breeds.
(I didn't word that well -- I'm not accusing vaccine developers of any kind of incompetence, just saying that for various reasons it's sometimes impossible to produce a vaccine that's completely reliable. I'm sure Dr. Doug or BCSnob could explain it better.)