Why is it....?

Possibly because of their past history as slaves. With the exception of Selene and a few handful, the vampires were the real villains. First they enslaved the lycans, then when they rebelled, they were hunted down. Selene and possibly many others were misled by the other vampires to believe that the lycans were all evil bloodthirsty monsters who were a threat to the world as a whole. This movie (and Blood Wars) is really the time where the lycans were the villains (with the exception of William).