Kenneth G. Wilson (1923). The Columbia Guide to Standard American English. 1993.
simple reason (that), for the
For the simple reason that adds emphasis, but it may also add an impatience or insult you do not intend: it can suggest that the reason is so simple that only a dolt could fail to grasp it. Unless you intend to be unpleasant, leave out the adjective, or, even better, say because.