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Well the original reason is we dont know what is the medium we operate in!
For instance,lets take one of the maxwell equation:
Del X (H) = (dow (D)/dow (t)) + J, where J represent the currents such as source current,drift curret etc which is added for consistency!
Now if u want to replace D by εE, then dow (D)/dow (t) = ε * (down (E)/down (t)) + E * dow (ε)/ dow (t).
We write like this because,we cant assume ε remains constant with time.. That is,the wave in one medium at one time may get into another medium at another time.. So we should not write directly like that!
Hope the answer wil be satisfying for you.. If not,then do ask me more..