I suspect the root is quite simple: it works. It's much easier to dig up some scary dirt on the other guy to convince undecided voters to vote against him than it is to convince those voters to vote for you.
It would be nice if someone were to come forward with a well-thought out economic strategy for the nation and try persuading the nation of its virtues ... but somehow, I can't imagine that working, or indeed imagine the public understanding it well enough to judge its merits properly.