Ethidium bromide in formaldehyde gel? - (Jun/28/2002 )

Simple question: What is the main reason why ethidium bromide is not added to the formaldehyde gel but rather to the sample buffer in Northern hybridization protocol? Thanks.

Once the gel is stained with EtBr after running, it takes long time until the bachground becomes clear (I used to stain for 10 min, then soaked in water overnight).