This might actually be the one case where this limitation is not entirely artificial. :-)

The SiriusXM sattelites are in an inclined eliptical orbit with the highes point being over the norther hemisphere. This is the so called Molniya orbit, named after the first Soviet sattelites using it:

https://en.m.wikipedia.org/wiki/Molniya_orbit

(Soviet Union had a lot of territory in higher latitudes where geostationary sattelites do not work well & the Molniya orbit needs less energy to achieve.)

So in the SiriusXM case, the sattelite would be much too low to provide useful coverage when over the southern hemisphere. And even when. As for not covering EU/Asia - this could be due to either the highest point of the orbit being always over US (guessing there) or due to ground station coverage when the sattelite is not over US.