There might be more popular eBooks by more popular writers about the same topic for the same or slightly higher price which people can buy at Amazon, ClickBank, eBay, e-Junkie, Author's webaite and other online market places and stores.
Lack of online reviews. You can ask some friends to review your book, and post their review of your book on their or your website.
Indeed, a preview would be nice for people who might plan on buying the book after they liked the first few pages or chapters.
There also might be a lot of free eBooks available on the same topic since a lot of people give away free eBooks to promote their affiliate products, products, services, and websites with embedded links in the free eBook which link back to the link they are promoting.
Also, it is very easy to find free pirated eBooks on Bittorent, File hosting sites, and peer to peer download programs.
Getting an eBook known is very competitive since so many people write them for free, or are more famous.