The later Priestly authors must have been uncomfortable with Moses’ Midianite connections, and what that implied—i.e., that Yahweh came from Midian! So what did the later Priestly writers do? They rewrote the tradition; they rewrote "history," and expunged Moses' Midian sojourn and Midian wife from history!
http://contradictionsinthebible.com/revelation-in-midian-or-egypt/