Aristotle doesn't mention Anaximander of Miletus in his rewiew of the predecessors in Metaphysics A. This study aims to show that Aristotle and Theophrastus mention Anaximander by name mainly in connection with Anaxagoras and Empedocles within the context of the coming-to-be.
For them the Anaximander's principle was similar to the mixture of Anaxagoras, which can be described as the one and manifold. Therefore, we may suppose that Anaximander is not mentioned explicitly because Aristotle regarded Anaxagoras to be more significant representant of the doctrine of coming-to-be as separation off of the opposites.
The other reason might be the indefinity of Anaximander's principle. However, the intermediate element in Metaphysics A might be interpreted as the Anaximander's principle.