Probably the snide reference to what the BBC "refused to mention" rule #0. Afghanistan has an even lower immigrant population than Japan, and rather stringent rules around public behavior, and few would call Kabul liveable. In terms of immigrant population, Japan sits around #155 of the ~230 countries in the world. You take a gander at the 75-80 countries [1] with less immigrants percentage wise than Japan (for a combined total by the way of 2 thirds of the world's population), map it against your own "livability" or "social trust" metrics and then see if your rule #0 (that the BBC apparently studiously avoids mentioning) still holds much water.

[1] https://en.wikipedia.org/wiki/List_of_sovereign_states_by_im...