They sing gospel the movie needed narration, so they made all narration done in song. So they made the narrators the demigoddesses of music, and gospel is the main style of music for continuing dialogue along with opera.
Why not opera whuch would be more thematic given the propensity of opera for Rome (which is closer than gospel in theme)? Because gospel is almost invariably happy music, great for a kids cartoon.
It can be argued that since they are depicted as living urn art for much of the movie, the black and earth tones help in the look of singing black women. WHO'DYA THINK YOU'RE KIDDIN', HE'S THE EARTH AND HEAVIN' TO YA-