The Sweden thing is a bit of a problem. As ESSB was their main airport until Arlanda opened ... But the size class restriction to class 4-5 airports has left Bromma out as it has been marked as sizeclass 2 or 3 airport, as the classification was based on today's passenger numbers, not what was going on in the 1950s.. In 1950s demand numbers Bromma would have had "plenty" of traffic, and been a size 4 airport at least. I have to change the size class system a bit later on.
So that's a kind of a bug. But wouldn't been noticed without this game world. And the apt size class restriction setting for game worlds is also a new thing and wanted to test that properly so that's why only 4/5 class airports were selected here.