Nosferatu wasn't even the first German horror film, actually. Both The Cabinet of Dr Caligari and The Golem: How He Came into the World came out in 1920, a couple of years before the vampire movie.

I'm not an expert in horror movies by any means, but I did do a study on stage theatricality in early German expressionistic films in grad school. Nosferatu was certainly a hit, and well-received in Germany upon release. The fact that most of the prints were ordered destroyed after Prana Films lost a lawsuit brought against them by Bram Stoker's widow probably added to the film's notoriety.

Hopefully, someone else can come along and give a better answer to the question.