Why does Malebranche feel that ideas can't be modifications of our minds?
Ideas are not generated by human minds because they are infinite. They can be applied to all objects and situations, so their sphere of application and validity is much larger than the one known to a single human mind. As the Enotes guide to the Dialogue summarizes:
the truths of the intelligible world are infinite, in that they apply to an infinite number of objects; hence, these necessary and unchangeable truths must apply not only to the limited, finite number of things in a person’s mind but also to what Malebranche calls “intelligible extension”—the entire rational world of concepts.
The "intelligible extension" is "the realm of true ideas" and must be located in an entity that is both outside and spiritually superior to the human mind, that is God. Unlike the structure of the human mind, therefore, the "intelligible extension" is eternal, infinite, immutable and necessary. This means that the human mind cannot question the validity of the ideas of the intelligible world. It is not up to the human mind to decide whether these ideas are true; as humans we simply have to accept their truth.