I think a Forgotten Realms Campaign Setting is a tricky book to get out.
First is the audience. You're immediately targeting only a fraction of the audience.
Most people play homebrew. 55% apparently.
From this:
http://slyflourish.com/2016_dm_survey_results.html
Currently, 38% use the Realms, but that could be skewed by the official adventures, and not a desire to play in the Realms, after all:
"The popularity of settings in the survey fell into three distinct clusters. Not surprisingly, our most popular settings from prior editions landed at the top of the rankings, with Eberron, Ravenloft, Dark Sun, Planescape, and the Forgotten Realms all proving equally popular. Greyhawk, Dragonlance, and Spelljammer all shared a similar level of second-tier popularity, followed by a fairly steep drop-off to the rest of the settings. "
From this survey:
http://dnd.wizards.com/articles/features/july-survey
And the catch is, fans of the Realms - by definition - already own the books. So you're selling to people who either don't have the books but are curious (a theoretical audience) or people who own the books and might buy updated copies (a potential/uncertain audience).
Especially since the setting is pretty much as it was in 1e/3e. There's very little changed now beyond the names of a few kings and some history.
And, by it's nature, the book will be incomplete compared to the expansive and comprehensive nature of the Forgotten Realms Wiki. WotC could spare themselves some time and money by just updating a few key entries there and directing people to that.