If I remember correctly both Mexico and Japan were interested in obtaining WRC seats when news of the possibility that Austrailia might lose their spot in the WRC tour.
Those two countries elected commitees to pitch why their locations would make prime suitors for a spot on the tour. Mexico had a rally event that Peugeot sent Harri Rovenpara to early last year and he raved about it. If I remember correctly, that's what got the wheels turning for that location. As for Japan, WRC and Japan have always wanted a rally there, with Subaru, Mitsubishi (and formerly Toyota) calling Japan home.
I think it all boils down to the fact that Mexico and Japan tried harder for seats than we did. Maybe they chose better locations than the US, maybe they indexed a lower operating cost than the US (very likely), maybe Mexico used the Carlos Sainz as a trump card. In the end, who knows exactly what factors went into it.
But look at it on the bright side, if you're a diehard enough fan, at least you don't have to cross the pond to see a live WRC event.