Don't you mean? :

"Democracy is the worst form of Government except for all those other forms that have been tried from time to time" - Churchill (allegedly)

Because the quote you gave doesn't seem to come from any known source.

The reason why any "pure democracy" is trash is because it's susceptible to the ”democratic fallacy”; whereby a majority opinion is misconstrued to represent some kind of "truth" and minority opinions are somehow "wrong" often leading to a form of potentially dangerous mob rule.