Why were the Tufts Beelzebubs used as the backup vocals for the Warbler's Songs?

I was just wondering why they used an acapella group for the backup vocals even though the actors that play the warblers (like curt mega as nick and ryker lynch as jeff) can all sing, yet they only lip synched most of the songs? (This is just something that's been bugging me and I can't find an answer anywhere)