It seems to me that one is assuming that God intended to make His word be affectual to all Mankind. i do not believe that Calvin has yet gotten in to aspects of the Divine Will. At this point, he has only maintained that God has given a (partial) revelation of Himself in the creation, and for a fuller revelation, one must attend to His special revelation, namely the Scriptures.
Calvin is not making assumptions in order to build a theology, he is taking slower steps, and attempting to avoid as many a-priori assumptions as possible. We could for example, question why God would give "general revelation" at all when He knew from all eternity that it would not be sufficient, or even why He did not make it sufficient, thereby eliminating the need for "special revelation".
But Calvin said earlier that God had intended for the general revelation to be sufficient, but Man's nature overrode what God intended (Calvin's thoughts, not original with me).
From Chapter 5:
Whether they will or not, they cannot but know that these are proofs of his Godhead, and yet they inwardly suppress them. They have no occasion to go farther than themselves, provided they do not, by appropriating as their own that which has been given them from heaven, put out the light intended to exhibit God clearly to their minds.