The idea of a vampire can be construed to be more than than what it stands to be. A metaphor for something that sucks the life out of you, for example, a tracker mortgage.
The idea of a zombie is also fascinating on many levels. The thought of something that is gone and dead forever, coming back with nothing in common with its former self than the bag of flesh it is. For loved ones in movies it is a hard connection to break. It is in fact a reminder that we are more than just this bag of meat we see before us. Another metaphor that digs its way into our sub conscious and creates a fascination? Maybe.
Overall, I believe that the fact that both creatures can be and mean more than what they are, reaches out to more than one audience. It appeals to us more than, for example, a banshee, which is more terrifying and interesting on the face of it, but a less popular mythical monster. Lack of metaphor may be its downfall, and it doesn't allow for much creative expansion.