Donald Trump’s rejection of the CIA's findings probably stems from the same root as his invention of 3 million phantom illegal votes for Hillary Clinton. Trump knows in his heart, or what passes for one, that a substantial majority of American voters rejected him, and that while he has won the electoral college, he has no true mandate from the people. His enormous ego demands that he disavow any fact which would diminish his victory in his own estimation, hence the number of imaginary illegal votes he dreams up conveniently exceeds the 2.8 million vote margin by which Hillary defeated him in the popular vote. Anything else which would cast doubt on his win, such as the Russian hacks, must also be denied in order to feed his fragile self-esteem.
On a side note, it is questionable how long Americans will accept the continuation of the electoral college system. This was originally devised to ensure that any President enjoyed broad acceptability across many states of the young union, not just in a few populous ones.
As the electoral map shows, this works – Trump won 30 states to Clinton's 20 – but America's demographics have changed dramatically since the system was put in place. As the country’s population becomes increasingly concentrated in a few heavily populated coastal states, such as California, New York, Texas and Florida, it is likely that electoral college wins which go contrary to the popular vote will become more frequent. We are already seeing the beginning of a movement to challenge the current system – "Not My President" – and Americans will increasingly ask why the will of the overwhelming majority of the people has to be frustrated in order to keep Wyoming and North Dakota happy. Watch this space.