Coca-Cola then became, and has since remained, the most profitable soft drink in the world. The "marketing blunder of the century" was so successful, in fact, that some people are convinced that New Coke Was an Inside Job– a delicately engineered gambit to revitalize the brand. As Coca-Cola president Donald Keough put it, "Some critics will say Coca-Cola made a marketing mistake. Some cynics will say we planned the whole thing. The truth is we are not that dumb and we are not that smart." Coca-Cola truthers have also suggested that New Coke was a disposable facade intended to mask the transition from sugar to HFCS.
There are several psychological explanations for the failure of New Coke, which are explored in this article at Damn Interesting. Link