Human embryo or dripping vampire fangs – you choose

I had an interesting comment from someone recently about the cover of my thriller 'The Immortality Plot'. I had been thinking it was pretty cool, summed up the underlying theme of the story and was a compelling image.
I used a human embryo.
I use the book and story evaluation site Wattpad, People can upload anything from a two-page story idea to a full-length novel and other people comment and if the book does well it gets read and the number of 'reads' it has is clocked up together with votes and comments.
It is a very useful site for a writer to test out new material. You can put up a book chapter by chapter if you like.
I noticed that  'The Immortality Plot' had around 700,000 reads which is pretty damn good, I reckon. But it only had 20 or so comments, all of them fantastic, praising the book.
Other books with similar reads were getting thousands of comments. I couldn't figure out why and I still can't. Then someone said the image on the cover might be off-putting.  I thought this strange. If people read the book then the cover is already in their mind. Why should it prevent them from commenting?
There is a hefty chunk of thrillers on Wattpad and a lot of vampire stories, tales of shape shifters, bodice rippers, horror stories, many with blood and guts type covers with a few fangs thrown in for good measure.
So I thought, why should an image as pure as a human embryo be more off-putting than a young Dracula lookalike?
Maybe you have a view on this. Wattpad is going to run a competition by getting one of its cover designers to design a new cover and then ask people to vote on the two side-by-side.
It should be interesting. I can't wait.
Take a look at a typical Wattpad page:  http://www.wattpad.com/user/DavidCallinan