Samuel Dravis gave a plausible explanation for this on the first page.
Originally Posted by Samuel Dravis
We're told that in the beginning, there was no evil - God looked at his creation and saw that it was (perfectly) good. But there's an idea here we don't want to miss: maximally perfect in the sense of limited beings is not maximally perfect in an absolute sense. Now, here I presuppose that God can't do logically impossible things, like making a REAL married bachelor and the like (I'm sure some would like to say he could but I'm going to ignore them). Anyway, created beings are by definition limited, so God is in a difficult situation - he can't create a being that's not limited, because doing so would break the definition of creation. He's left with beings that are limited, and being so limited they are capable of error (and thus sin).
Perhaps you are suggesting God should do something logically impossible?