In C’est facile à faire, c’ replaces a previously mentioned subject (its antecedent). In this type of construction, the preposition has to be à.

In C’est facile de le faire, c’ is a dummy subject – it’s not replacing anything. (Just as when we say It’s raining, “it” doesn’t replace anything.) In this example, le is replacing the antecedent. When you have a dummy subject, the preposition has to be de.

This is just one of the particularities of French impersonal expressions.