Why are bacteria divided into two domains?

this is to do with the process known as Gram-Staining.

basically gram-negative bacteria do not retain crystal violet dye and gram-positive do.

gram-negative have an outer leaflet of the membrane that comprises a complex lipopolysaccharide whose lipid portion acts as an endotoxin where as gram-positive do not; this is helpful to know when treating ailments and diseases in animals.