Parmandur
Book-Friend
More of a quibble, but you can honestly make any setting a sandbox based on how you use the printed material. I've made Forgotten Realms a sandbox with Phandelver/Icespire Peak.
So the argument here that "Nentir Vale is a sandbox and Exandria is not" is a little silly because both easily can be, it's a style of play.
What @Aldarc really seems to be arguing in favor of Nentir Vale is that you can more easily fill create your own details to fill in the many available blank spaces of its lore. Which is indisputably true, but doesn't make it a better or worse sandbox than any other setting (which is really down to the DMs playstyle).
Well, precisely: that's how I've always experienced Greyhawk or the Forgot Realms, and how Perkins has utilized sections of the Sword Coast, Chult and Barovia to set up little sandboxes for each big Adventure product.
But that cosmogeny, explaining how the world came to be, weirdly lacking and phoned in for Greyhawk and the FR for all their other details. Nentir Vale has that covered very nicely with the Dawn War.