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:
(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.
In 2016 Sirius discontinued broadcasting from tundra orbits. Their newest FM-5 uses geostationary. [1]
Satellite radio was tried a few times in other geographies, but only in North America did it succeed. [2] In 1999 Worldspace launched for EMEAI, and looks like it collapsed in 2008. MobaHo! was a mobile satellite digital audio/video broadcasting service in Japan whose services began on October 20, 2004, and ended on March 31, 2009.
Interesting, I totally missed that they use GEO now!
Still pretty cool that one can receive digital radio without a directional antena from a satellite all the way up 36000 km in GEO. Though looking at the article about Tundra Orbit you have linked it was 25000 x 39000 km already, so not that much of a difference.
And in addition population density is higher in Europe than U.S., menajng that the density of FM radio (and modern attempts like DAB/DAB+ etc.) and mobile cell base stations is also higher, thus those provide more competition.
