Their design goal was to unify the Forgotten Realms fanbase as much as possible. They discovered that metaplot did the same thing to their fanbase that publishing a number of Settings did: every time they advanced the world, they lost a slice of customers to those who stuck with the previous timeline, rather than growing the overall numbers. Ao, SCAG is meant to work with 1E, 2E, and 3E Setting products, or appeal to those who wanted to keep "up to date." Then, cut out the metaplot, and let the Setting stay stable.
They really want people using the 3E FRCS and the 1E Grey Box to be able to run the official Adventures in 5E.