My understanding is also that 978Mhz was picked (or rather, anything not 1090Mhz) because with the population of aircraft in the US, 1090Mhz would have become saturated (because ADS-B is always transmitting, not waiting to be interrogated). With fewer aircraft flying in most of the rest of the world, that wasn’t going to be the same issue.
But as it turned out, a lot of US aircraft got dual-frequency boxes, so they’re effectively using both frequencies. I don’t know how that affects the frequency saturation issue, or if that even was actually an issue, though.