Then consider the risk of a sequel not selling well. The financial consequences would be pretty severe. DLC on the other hand involves no risk whatsoever. A song doesn't sell well? So what! It didn't cost much to have it available in the first place, they already have access to most of their discography.
Maybe a track pack of 20 songs already available as DLC makes sense, but a full game? They need to move on to other projects.