Because fronted "Not only" triggers inversion. The non-inverted example in option A sounds strange to the crushingly overwhelming majority of native speakers of any variety of English. But we should not outright
dismiss option C for the reason mentioned in the next paragraph.
While the preposed adverbial construction "Not only do Polyergus ants..." (Option B) does not have exactly the same force as the cleft¹ construction "It's not only that Polyergus ants..." (Option C), it nonetheless seems to me—I haven't pondered this too deply—that clefting preferably serves the communicative functions of explanation and disinterested contrast-drawing. IOW, we might rewrite the sentence in option C as follows: "It's not only that Polyergus ants subjugate ants of the genus Formica.
They come to
utterly depend on them for their subsistence." Of course, someone else may rewrite it differently.
¹ Those interested in learning more about "cleft sentences/constructions" may find this article