Complex sentence: Public interest in contests was in decline from the 1950s but Besses was one of those bands that had previously diversified into recording their output also and sales of those recordings continued to be significant for another decade.
Split into simpler sentences: Public interest in contests was in decline from the 1950s but Besses was one of those bands that had previously diversified into recording their output. Sales of those recordings continued to be significant for another decade.