The financial Wizards of Oz who select presidential candidates for both U.S. political parties won’t allow a serious third party candidate to run as an independent.
This came about after Oz put Ross Perot on the 1992 presidential ticket as an independent candidate with democrat Bill Clinton and republican George H.W. Bush, Sr.. Their reasoning for this was they’d vetted and decided Clinton would be their next president, but Oz knew he couldn’t muster enough votes in the general election to beat Bush. So they ‘asked’ Perot to get in the race to bleed enough conservative votes away from Bush for Clinton to win.