I’m taking this as a question about books in general (it doesn’t ask why aren’t Alexander Boast’s books bestsellers). Secondly, I’m taking it as meaning a genuine bestseller, not something that tops an Amazon best sellers list for 3 minutes on an afternoon.
People in the overtly artistic/creative world tend to have an exceptionalist view of the work they do and the products they sell, but the truth is, in respect of their commercial behavior, books are no different to any other product in terms of basic economics. How many are sold is a function of supply and demand.
In the days of ebooks, the supply of books at a given price is infinite.
Demand has three components: The desire to pay, the willingness to pay and the ability to pay. You need all three to make a sale.

Desire to purchase

Without the desire to purchase amongst consumers, you have no sales.
If a book is based around an established franchise or author, there may be latent desire in the market.
Desire can be inspired through positive exposure: media promotions, celebrity endorsements and most importantly of all prominence in bookstores (online and offline).
Desire will be reinforced by a good cover and blurb, but this is only relevant to those who are exposed to it. A good cover and blurb without exposure are of no value.
If the content of the book resonates with a large chunk of the book-buying public (largely a matter of being the right content at the right time), word-of-mouth recommendations may create additional desire through a snowball effect. As with blurbs, this is a multiplier effect, so it’s only of big value if the starting level of exposure is reasonably high.

Willingness to purchase

In order to persuade someone to purchase, the book has to seem a better an option than other ways in which the consumer could spend their money.
Therefore a book has to be priced correctly for its market, i.e. in relation to other books that the same customer might choose. In some segments of the market (e.g. fanfiction, poetry) this price is zero or close to zero. In others, it is higher.
On top of the book has to have a better cover and a better blurb, a better first few chapters and better endorsements and word of mouth than the best alternative product available at a similar price.

Ability to purchase

Is probably the least relevant for books because of their low absolute price point and high relative affordability, but needless to say, if you price your paperback at $10,000, you probably won’t sell many.
A book which is not a bestseller will be lacking in one or more of these qualities. A book which many people want to buy can afford to buy and are willing to buy will sell in large numbers.