Complex sentence: BMG continued to release her band's CDs in Japan, but remained skeptical about releasing the music in the United States--since the 1950s, big band music has rarely achieved commercial success in the US.
Split into simpler sentences: BMG continued to release her band's CDs in Japan, but remained skeptical about releasing the music in the United States. Since the 1950s big band music has rarely achieved commercial success in the US.