Populist policies are about engaging people who felt left behind and cut off from politics. They are about mass appeal to large groups of people.
Democracy is dominated by elites. Many central political parties do not represent the views of large demographic groups. They felt like they had no one to vote for who would speak for them, their values and anxieties. The popularity of populist parties and politicians shows there was a hunger for this type of politics. Elite consensus has held for decades that these views were not viable or sensible, and so they were ignored by the mainstream. The fact that these views have gained so much shows that they existed but were not represented. In a democratic world, representation of views is something we should value. Whether or not we agree with them, they should be openly discussed. Everyone has an equal right to vote and be represented, and the fact that a few elites disagree with them is no reason to stop it.
Populism threatens democracy. While its greatest beneficiaries may be using democratic process to increase their own power, they do not intend to keep these channels open for others to compete once they are elected.
People's views should be represented in a democracy. Populism represents views of people. These views were not being represented.