"People vote for optimists, not pessimists, even if that means obscuring hard truths in the short term."
That's it in a nutshell.
Offer a Pleasant Illusion, blame someone else for the Difficult Reality, and you get elected.
Luckily the meat-handed mouth-breathing electorate will not hold you accountable five years later when you get around to compromising your position, or fifteen years later when the economy craters because of your delusional decisions.
We're just feeling the Clinton fallout now.