You shouldn't forget Tal'Dorei was published as setting by Green Ronin. This means Exandria wasn't an original product by WotC.
Now WotC is working for the perfect crunch, but the fluff/lore/background is practically frozen. If something we learn with 3.5 Ed is to add new crunch (feats, classes, spells, PC races, monsters, game mechanics..) is harder to be added to previous settings... I guess WotC will continue the metaplots when they start to make enough money with the multimedia projects (videogames, cartoons, some action-live production, comics..).