He generalised his faces, composing an ideal cast of features out of several types.
In the rendering of the face and head, then, he chose to be a Classic, while in the treatment of the body he was vehemently modern.

In all his work which is not meant to be dramatic--that is, excluding the damned souls in the Last Judgment, the bust of Brutus, and some keen psychological designs--character is sacrificed to a studied ideal of form, so far as the face is concerned.

That he did this wilfully, on principle, is certain.

The proof remains in the twenty heads of those incomparable genii of the Sistine, each one of whom possesses a beauty and a quality peculiar to himself alone.

They show that, if he had so chosen, he could have played upon the human countenance with the same facility as on the human body, varying its expressiveness _ad infinitum_.
Why Michelangelo preferred to generalise the face and to particularise the body remains a secret buried in the abysmal deeps of his personality.