I'm not so sure TBH. Professional writers want to get paid, and if WotC wanted to commission a new FR campaign book then I refuse to believe they have nobody on staff or on retainer with sufficient love and enthusiasm for the Realms to take the job on. The vast amount of 5e FR material on DM Guild (including from respected pro writers) certainly argues there's still a lot of people out there who'd love to write the Realms.
It really seems a deliberate strategic choice at this point. FR is treated as the 'default' setting in campaign books (a bit like Greyhawk was in 3e), and gets a bit of fleshing out piece by piece in those, while new campaign books are largely those with a pre-existing built-in audience that WotC are trying to leverage - MtG players, the Critical Role audience, or Eberron players.
I think as WotC looks increasingly to other settings, the plan will change. Plus doing FR by piece meal isn't working when you've left the Swordcoast for another part of FR all of once in what 5 years?
It's way, way to slow.
I think the game plan at WotC has really changed, they have a huge pile of APs between official and Adventure League content, and for like years they wouldn't touch campaign setting books at all, now by June they will have published 3 in a row! Four total in a short span (and no Curse of Strahd doesn't count, it's an AP regional book at best, and the SCAG is also a regional book), so it makes sense that they would be more open to a 5e version of the FRCG now, plus once that is out they won't feel like they have to set so much stuff in FR.
And no the Forgotten Realms isn't the default setting, although it's an understandable mistake given how reluctant they've been to visit other settings until 2018, the D&D Multiverse is seen as the default setting.