Quite simply because it's something to bicker over and try to reaffirm one's own beliefs which in this case happen to have no solid proof by which to support those beliefs on either side no matter how much they blabber on. This is not unlike many other "issues" that politicians/media/etc. bring up about various candidates.
Technically one could argue that the educational opinions of anyone in D.C., much less the White House, has no bearing on anything whatsoever since public education is the responsibility of State and local governments and not that of the Federal Government. The mere existence of the Dept. of Ed. itself is constitutionally questionable. From what I recall there is no mention of education in the US Constitution so Congress has not been given any such authority. Meanwhile education is mentioned in at least most, if not all, State constitutions.
What some candidate thinks regarding the origin of our species, the existence of aliens, which OS is better, etc. is completely irrelevant. But of course candidates views on issues which, if elected, they will have the ability to affect are ignored. Nothing new here.