The main answer is "minorities". Look at the American South during the Jim Crow era.

Minorities need protection from majorities in democracies. And not just racial, religious or ethnic minorities. Everyone is a minority in multiple ways.

You have hobbies, beliefs, goals and connections that the majority in your country doesn't have. And they are all at risk under a democracy. This is why we don't have a "pure" democracy. This is why the Founders put many anti-democratic features in the US Constitution.

Every right you enjoy is a point where society drew a line and said "this will not be subject to a vote".

For one, it would be a terrible choice for a charity dedicated to helping the poorest of the world. People in democracies tend to vote for their own interests.

The U.S. spends about 1% of their budget on foreign aid. Yet in polls, most Americans think foreign aid should be further reduced [1].

[1] In the following poll of 1012 Americans, 81% thought foreign aid should be reduced and only 18% thought it shouldn't be: http://i2.cdn.turner.com/cnn/2011/images/01/25/rel2d.pdf