Because populism leads very quicky to dark places. Most all systems of government, of vested authority and state monopoly of violence, are to address our instinct towards populism. Egging on the mob never ends well.
You're only labeling it "populism" (which is normally defined as 'support for the concerns of ordinary people') because you don't like the outcome. If you liked the outcome you would call it "democratic."
Somehow the idea of being concerned about the outcome for ordinary people has become vilified by American liberals because red state, blue collar, ordinary people are worthless.
What is going on?! Trump wants to be a job protectionist while Democrats want globalization? Aren't you people aware that not that long ago the Democrats were fighting the Republicans against globalization because they claimed that globalization was only good for corporate interests and bad for ordinary Americans?
PS I think Trump is a nut job in many ways so please don't read the above as an endorsement.
The populous isn't always in the best position to think through the long-term implications of their desires.
In this case, Trump rode the anger of voters who want to close the borders to both immigrants and imported manufactured goods, because they perceived that some of their jobs going to illegal imigrants, and other manufacturing jobs had been taken overseas.
How many of those people, do you think, buy everything at Walmart (whether it's all they can afford or not)? How many of those people realize that a 35% border tax and getting rid of our internal slave labor force will lead to commensurately higher prices of goods and services, without necessarily bringing back the jobs that were actually automated away? If they have thought it through, where's the better outcome?
This is why we don't have a direct democracy. These issues are more complicated than one can wrap their head around without making it a full-time job for an industry, and the decisions aren't helped by folks making decisions out of anger and desperation.
Populism is, in a political culture, the appeal to the popular regardless of other interests. It is about simplistic concepts easily chanted, divorced from complex realities. It is about doing what the people say they want regarless of how idiotic that may be, regardless of the fact they probably dont want the thing in the long run. By way if example, tieing NAFTA negotiations to whichever country wins the Stanly Cup would be popular, but it isnt sound leadership. Eliminating the IRS is popular, but no rational person would do it. (Either of those wouldnt shock me if tweeted.)
It's not exactly a citation, but high school culture ("that which sounds cool should be done") doesn't exactly lead to the best of all possible places.
Looking for symmetric universal moral principals sort of does seem to. Though I admit basically no form of government seems to head in that direction, as founding documents post-revolution are not really a form of government in my model.
