Some of it is due to man's propensity to flaw, and to his limited ability (which is related to his limited mental and physical power). In addition to misinterpreting the nature of the relationship between intellect and free will, Descartes has incorrectly interpreted some of the most vital connotations that accompany free will. There is an innate responsibility that accompanies this gift. Free will presents human beings (and anything else endowed with it, for that matter), the opportunity to do good or evil, to make use of or to squander opportunity, to laugh or to cry. The power of the decision, regardless of the source (which is, of course, God) ultimately resides with the individual. And while the author readily acknowledges the relationship of intellect and will in whether or not man can judge correctly or incorrectly, he does not acknowledge the fact that there is a responsibility associate with such acts that does not reside with God, but of the person making the decision. Due to this fact, one of the author's central premises in this meditation -- that a perfect being has created man so man should be perfect in all his actions and judgments -- is flawed. God's gift to man of free will allows the latter to demonstrate his own actions and decisions, which enables man to operate as the authority for such actions. Therefore, man must demonstrate his own savvy, his own good judgment and the research required to do so, in order to properly discern "truth from error." The very creation of free will indicates that God has delivered man the tools to either succeed or fail, and reinforces the fact that doing so ultimately resides with man himself. That is the true nature of free will, yet this fact is never acknowledged by the author in this work.

Descartes is not expressly wrong in his attribution to the source of error that is frequently found in man. His theory has some veracity regarding the source of this error. However, the author's...

There are other causes than this one, such as the fact that free will is hampered by man's lack of intellect, and is not nearly as unlimited as the author claims it is. Additionally, man's ability, his prowess and proficiency at performing certain actions such as decision making and rendering judgments also factors into why human beings make mistakes. Perhaps even more significant is the fact that the author does not recognize the fact that the very bestowment of free will readily transfers all authority and responsibility for resultant actions upon the nature of man. By the very definition of this concept, then, man can do what he has the ability and intellect to do -- which certainly includes failure, error, and a host of other negative behaviors that in no way reflect the creator, only man's limitations.
In summary, however, the actual reason that Descartes' account for the errors that man makes does not work is due to this methodology. He renders a number of fairly dangerous assumptions -- such as the fact that since man was created from a perfect being, he should be perfect himself. Yet he also mistakes the relationship between intellect and will and the limitations that the former presents upon the latter. Ultimately, however, the author is incorrect in his very conception of man as a whole and the role that free will plays in his existence. Man has the potential to render perfection in his being and actions due to the endowments presented him by the creator. Yet humanity is synonymous with error -- that fact, more than anything else, is what being a human essentially is. Free will is what makes this possible. If there was no free will, then man could be as perfect as he wanted, and do strictly what God intended him to do.