I dont have a lot of economics experience but im in a business major, from the best of my knowledge, the answer to the question is the concept of equilibrium. For a market be in complete equilibrium the quantity demanded must equal the quantity supplied. As a result, to keep the market at equilibrium, the income elasticity of all goods in general must be equal to one. Accordingly, if one good has an income elasticy of demand over one, one or more other goods must have an income elasticity of less than one to bring the general income elasticity back to one. Therefore equilibrium is achieved and the demand and supply remain equal.
im not 100% sure of the answer but i guess somebody can verify it?