G. k. chesterton believes that saint thomas aquinas viewed man as a whole rather than as a beast like the anthropologists. why did he think this?

Perhaps Saint Thomas Aquinas was more attuned to the moral wonder within (the true man as reflecting God's image and likeness) and the natural wonder without (Natural law). Perhaps he lived in a time, and in a place (a monastery), which encouraged the attunement. 

A scientist (one who cultivates control of natural forces) is more inclined--as Kant observed--to leave aside God, morality, Soul, and suchlike, focusing only upon that which can be materialistically measured, quantified, and controlled. Many inventions, not so much development of soul-field awareness. 

Related: "God at the Speed of Light."

I hope this helps. C: