Think of any three populist movements, and chances are at least one of them trades in xenophobic rhetoric. Targeting voters' fear of the unknown is a straightforward and powerful tactic, that is dangerous.
Populists spring up in times of unrest and economic downturn, when the mainstream political establishment has failed to deliver on its promises for prosperity. They promise a new way to solve problems. The issue is that this promise is often radical, new and exciting. It appeals to people's most basic fears, and often those fears are directed towards people who are different. It's basic human instinct to distrust that which you do not know. When we were cavemen this proved useful because we wouldn't eat foreign poisonous berries. Nowadays however, that mistrust is directed at people's whose lives or cultures we don't understand. This is particularly true when we ourselves feel threatened or insecure - like in times of economic downturn or rapid cultural change. It's only natural then to see populists playing on this fundamental distrust of the foreign at times when people are frightened for their futures and want an alternative. The issue is that these fears get wrongly directed towards those who are different, be that for reasons of race, sexuality, or religion. The truth is that no one minority group is ever responsible for the loss of jobs or changing societal mores. Minorities are just as much citizens as the members of the populist majority, yet because of their status as a minority have less political power to defend themselves. The result is their victimisation and scapegoating, which is unacceptable in a society that prides itself for equal treatment of all.
Some populists do use anti-immigrant rhetoric. But this is not true of all of them. It can be equally powerful to mobilise people against a ruling elite that they see as having failed them. In those cases, holding powerful groups to account can actually result in tangible change and improvement of a democracy.
Populists are successful because they appeal to people's frustrations with the current system. The easiest way to do that is by turning their anger towards a group people are already mistrustful of. Majorities are often mistrustful of minorities. Populists often turn people's frustrations towards minorities.
Rejecting the premises
The easiest way to do that is by turning their anger towards a group people are already mistrustful of. Two issue with this: firstly, this may not be true. It is equally easy (and perhaps more sensible) to turn the blame towards the elites - who have been in power and are the actual source of frustration for many people. Anger at elites would not be either unnecessarily damaging or unfair, given that it is often for their poor choices that the majority has to suffer. The second response is that even if it is easiest, that does not necessarily mean that is the path taken by populists. This premise relies on the assumption that populists will do all they can to get elected and seize power, when in reality they often are motivated by an actual desire for change.