How is it overdone, please tell me how?
Oversaturation. Between movies such as Underworld, Van Helsing, Twilight, and television series such as True Blood, there's been an overabundance of Werewolf v Vampire things in the last decade, making any new attempts to do it seem like jumping on the band wagon.
There's no reason it COULDN'T work, but it's going to be hard to bring something new to the table in a time that everybody seems to think the key to big money is combining these two monsters.
It's the same oversaturation with Zombie Apocalypses. Zombies have infected almost every game genre imaginable, as well as books, television shows, podcast dramas, and film to the point that I almost always groan when I first hear about a new zombie thing coming out. Some people make great use of it, such as Rockstar Games with Red Dead Redemption: Undead Nightmare, TV series such as The Walking Dead, and the podcast We're Alive! However, just hearing something has zombies in it doesn't make me want to experience it right away anymore because too many people are trying to go back to that well at once.