Two factors in why songs are seldom in dialect is to do with what the collector felt and understood, and the intended audience. My favourite dialect issue is in "English County Songs" by Lucy Broadwood. One of the northern songs refers to 'marra', which is term usually meaning workmate but often a more general friend. This is rendered as 'marrow', so we have the wonderful line:

As me and my marrow were going to work ...

which conjures up an image of a miner with a large green vegetable under his arm.