Keep in mind that this is just a working theory. But it seems that with some worlds, there's access to different sort of sub-worlds as well, those that I call realms. For instance, on my Earth, there's magic in place so I can access the realm of faerie and travel between the two easily. However, the Earths of other carnival members have nothing of the sort, even if they have magic in the first place!

It's possible that with the world of Portland, the Severing cut off access to other realms, such as heaven, hell, and faerie, as well as other worlds that work in a drastically different fashion such as Atlantis, Alola, or the world of the Mainframe.

[ the more he talks and the more he hypothesizes about this...the more Strange just goes into a giant nerdy rant about this. ]