Fear mostly. Fear of being labelled a racist by the liberal mainstream media. Roger Ailes shut down any dissent from commentators on FOX news.
Fear? Several thousands-or-so might fear media retribution.
But that leaves more than a few million who a.) knew that Obama's father was not a citizen and, since it's so obvious now, b.) should have known this automatically made him ineligible.
Why didn't anybody blow the whistle?
Not everybody reports to Roger Ailes. Not every constitutional scholar or law professor is a liberal.
In actuality, isn't it more likely that nobody in a position to know thought it was a disqualifying circumstance?