Complex sentence: By the 1950s, Afro-Cuban jazz had been using modes for at least a decade, as a lot of it borrowed from Cuban popular dance forms, which are structured around multiple ostinatos with only a few chords.
Split into simpler sentences: By the 1950s, Afro-Cuban jazz had been using modes for at least a decade. This was because a lot of Afro-Cuban jazz borrowed from Cuban popular dance forms, which are structured around multiple ostinatos with only a few chords.