- Setting books have proven to be lucrative in the 5E era.
- It is a way to tie together all/most of the story arcs that have been published so far, especially for the "5E newbies."
- Continuing on from the last point, it facilitates "adventuring beyond the story arcs" for those newish players who want to build their own adventures.
- Because its well past time to publish a real setting book for the default world of 5E D&D. For better or worse, it has been the primary D&D setting for the last three decades and deserves a new book in the New Golden Age of D&D.
- Because it wouldn't be hard to do, given all of the available material.
- Because the people want it.
- The 3E FRCS is considered by many to be the best campaign book ever published, so they'd have to find a way to at least equal its quality, which might be hard to do.
- Unlike other new (and successful) setting books, there are mountains of Realms books available, so the market might be smaller than one might think. Maybe.
- It would require making some hard choices about the world in the 1490s that haven't been necessary with the vague hand-wavyness of "It is kind of back to 3E era, but with a few differences."
- Could be seen as anachronistic and too 80s-90s for the contemporary fan-base.
- The success of 5E may partially be because it is so focused on distinct story arcs, and a setting book implies an approach adventuring beyond the confines of published stories and/or sandboxing that might not fit the current ethos.
- "Enough of the Realms, already!"