The answer is simple. It's because of the lack of understanding from the masses.
The confusion begins when we listen to a politician speak. Without knowledge of what the politician is doing through his speech, we cannot understand what he is saying. Politicians use a wide range of logical fallacies in the speeches. you will find that a politician never commits to anything, no promise is actually made, However, our minds logically add a conclusion. Thus, logical fallacies like circular reasoning, begging the question, ad hominem, red herring, can be used so effectively since the masses are not taught to identify them unless they are educated in critical thinking. Consequently, when the people put their faith in a particular politician/party, and sees no evidence of his/her alleged promise being kept, The people become angry.
A great example of this is Donald trump. He uses Ad hominem fallacies to create an emotion with the public, however, there is no logical reasoning that is not fallacious present in his speeches. Circular reasoning is the type of reasoning that requires the conclusion to be true, to support the truth value of a premise. An example is:
Mom: don't do that again.
Mom: because i said so.
Here the mom has provided no reasoning as to why the child should not do that, thus its circular. We also see this a lot in religious texts. E.g i know god exists because the bible told me so.
The red herring distracts you from the original issue, which is another ploy the politicians use.
Then there is the media, that inflict us with biased opinions and persuade us to a certain way of thinking.
Ultimately, democracy doesn't fail, the leaders do. The idea of Democracy is a fair and just society for all its civilians, but our leaders manipulate and corrupt the system by using trickery to fool the masses into believing they genuinely care about YOUR well being. A politician simply is an employee, he does his job the way that it has been done by many before him and by many more to come.