Presumably because he seemed that much more like a god than like a man. 

Our present understanding of heredity was not yet available, and absent that, the proportion between the genetic inheritances from father and mother was pretty much anyone’s guess. In the *Eumenides* of Aeschylus, the god Apollo [claims the father’s share is 100%][1]! (The chorus of Furies does not buy it.) Sterne in *Tristram Shandy* (Vol. 1 [1759] Chap. 2) mocks the somewhat similar theory of the spermatozoon as containing a miniature but complete version of the person to be begotten, the *homunculus.* 

Even *with* the benefit of modern understanding of chromosomes, meiosis, and fertilization, the notion persists that one can be one-quarter Czech (or whatever), because exactly one grandparent was supposedly pure Czech, despite the obvious arithmetical fact that the number of chromosomes in our species’ somatic cell nuclei, forty-six, is not an integral multiple of four. This obsolete notion is even written into [current law][2], with current real consequences, as applied to membership in recognized Indian tribes within the United States.


  [1]: http://www.perseus.tufts.edu/hopper/text?doc=Perseus%3Atext%3A1999.01.0006%3Acard%3D640
  [2]: https://en.wikipedia.org/wiki/Blood_quantum_laws