Why is clear enough from a basic point of view. In "un secteur du numérique" the word "numérique" is a noun, a well defined singular noun which has been coined to name an industry. In "un secteur numérique", the word "numérique" is merely an adjective, and so the whole industry, as a concept, is not immediately made salient in the listener's mind. The adjective could in fact be used in relation to numbers in a general way, not specially binary numbers as they pertain to the processing of information, so this latter  term has not as much individuality, so to speak. I see that as one of the reasons, but perhaps more can be said.