I guess because it is the lowest-level category. For instance let's say we have a category for emigrants from Södermanland, Sweden to Minnesota, USA. The category would contain exactly the same profiles as the symmetric category Immigrants to Minnesota, USA from Södermanland, Sweden. So instead of duplicating the categories, only one is used "Migrants from ... to ...".
This is really excellent work, Steven. I am amazed a how simple it is, in the end, considering the complexity of the premise.