Explanation:

If you say that God came in the form of a Fish, Tortoise [1] etc. also, which are not Human Incarnations and hence if you argue that God cannot be confined to entering only the medium of a human form, the answer for this is given here. Even in the Fish and other incarnations a soul exists in a materialized gross body. Hence, our statement that God enters a soul that is associated with a materialized gross body need not be contradicted. By this, the possibility of entry of God into inert objects, which do not have a soul, is ruled out. Now, after establishing the fundamental concept of the entry of God into a soul in this manner, and after negating the entry of God into inert objects without a soul, we propose another alternative concept in the case of the above-mentioned incarnations such as the Divine Fish. The incarnations such as the Divine Fish came just to kill demons and not to conduct any preaching of spiritual knowledge. Hence, instead of God, only the power of God, which can kill demons could have also entered the Fish and similar incarnations, by which the divine purpose is fulfilled. Now, we can say that God enters only the human form for preaching spiritual knowledge, since the human form alone can preach the spiritual knowledge.

[1] Incarnations of God as the Divine Fish (Matsya), Divine Tortoise (Kurma), etc. mentioned in the Puranas.