RJ just said that it plays a role, if a person is at SG, because the DO has basically ultimate power there. That makes sense and we already saw this, e.g. regarding Fain becoming the DO's hound or mindtrapping there. It doesn't indicate, however, that the distance to SG otherwise plays such a role in the transmigration/balefire issue.

Distance can especially not be the answer, because RJ and BS mentioned that they won't reveal such other limits yet, since they might use them as surprises in the coming books.

And if those other things actually played a role, like that the Shadow had just experienced a major defeat in the end of TFoH, then it would actually be the combination of three (or more) factors, which prevented Asmo's transmigration, not only two as RJ said (bascially how, where AND when he died).