Promotional literature, in this case, means literature intended to promote a commercial venture. The first permanent English settlement in America was sponsored by a private joint-stock company: the London Company of Virginia.
This company received its official charter from King James I in 1606, sold shares to the public, and used the pooled capital to outfit ships and supplies for the proposed English colonies in North America. In Renaissance England, wealthy London merchants (and members of the nobility) were interested in investing their capital in foreign trading ventures.
The company selected Sir Thomas Smythe, a wealthy merchant and former governor of the British East India Company, to lead the Virginia Company. Because the national honor, the reputations, and the fortunes of many high-profile investors (including the king and leading nobility) were at stake, the company was granted certain privileges to protect its interests. One of these privileges was that all printed materials about the venture were to be approved by the company before publication. That way any inconvenient information that would tend to discourage investment or settlement could be handled with care or simply edited out.
It appears that this is what happened with Sir John Smith's "The General History of Virginia, New England, and the Somers Isles" because in addition to being history, it was also intended to promote further sales of stock in the company. For this reason, the merits of the settlement were likely emphasized, while the negatives tended to be downplayed.