Answer: Probably because they are copyrighted material. Quoting song lyrics does not fall under "fair use" as it's currently defined (and there are as many definitions as there are lawyers, but you can avoid all that court stuff by removing them). To be safe, you should never quote from any copyrighted material directly without permission in writing from the publisher (not the author). Your book publisher could get (and pay through the nose for) such permission, but it's expensive. Undoubtedly your gal isn't likely to take on such an expense for the sake of quoting the songs.
There are ways around it, however. You can just tell the reader the characters sang whatever song -- it's okay to use the title. Or you can easily paraphrase and have your character sing along with Kris Kristofferson“about being broke and hitching rides in Baton Rouge.” But you cannot have the character sing the actual words of Bobby McGee, as in , “Busted flat in Baton Rouge, headed for the train...”
It’s okay to quote from songs published before 1920, because that material is in the Public Domain. Dead songwriters cannot sue but their music publishers will certainly do so. And the Musician's Union will likely foot the litigation bill. Basically, it’s okay to paraphrase song lyrics, to quote song titles, or book titles, or movie titles, to mention celebrities by name (they are public figures), but not to copy directly from any copyrighted work and have it reproduced in your own.
Hope that helps.