The reason why I think these places will be used in future Fallout games is several fold
1. Even using a Fallout 1/2/Tactics style map that covers a are area of land, we would have needed like 30+ games to cover all the U.S.
2. Now that each game is focusing on only one city at time, it would take even more.
3. While there are plenty of big cities in America, not all of them have the same level of historical or cultural value.
4. Even with many of those that do, they are so close together, especially along the east coast and the eastern Midwest, that it would take forever just to get out of one small area, and all of those games were have such similar style of architecture and culture that they would all be basically the same.
So with all these in mind, the developers have to pick and choose cities of high cultural/historical importance, that are far enough away to be different then other cities, but also close enough.
If we look at the east coast for example
-Boston
-Washington D.C.
-Charelston(Likely the Broken Banks)
-Miami
-New Orleans
Are all roughly equal distances from each other, and cover most of the east coast.
With the Vanilla games they can show these cities, and the wastelands immediately surrounding them, so we can get a picture of what the wastelands and larger cities of the region are like, and then they can use DLC like Far Harbor, Point Lookout, and Honest Hearts to show us what the more backwoods areas are like, and DLC like Nuka world, The Pitt, and OWB to show us how other pre-war locations/towns have been doing since the war.
With all of this we can get a somewhat complete picture of how this region is, without having to actually see it all.
Fallout 4 + DLC covered New England, states like Massachusetts, Maine, Vermont, Connecticut, Rhode Island, and New Hampshire.
Fallout 3 + DLC covered Maryland, Virginia, West Virginia, New Jersey, Pennsylvania, Delaware.
Fallout 5 + DLC will likely cover the Carolinas + Georgia