The general population fears the unknown. Since the dawn of politics this has been true, and is no secret to those vying for positions of influence. When it comes time to vote most people do not research, I mean really research, the candidates. Voters rely on common advertisements, media, friends, political debates, etc... for background. And for those that do try to research there is an overwhelming amount of unverifiable information emanating from dubious sources so that one is left rather dazed with hardly enough time in the day to sort through it all for each candidate. Hardly a matter of circumstance or chance. In the end (or the voting booth, to be accurate), given a candidate that is familiar vs. a candidate that is not then the safe bet will almost always appear, even given a candidate's bad track record, to vote on familiarity. That is why campaigning works; why an exorbitant amount of cash is spent on campaigning; and why alternative parties with little campaign funds are excluded in debates.
Simply, the average voter is ignorant of the issues, and the formats of campaigns don't generally allow constituents to grill candidates sufficiently. Questions from the audience in debates and town hall meetings are generally vetted, and politicians often skirt the question and give half-assed or indirect answers, with no ability for the questioner to follow up. This kind of format makes it nearly impossible to get an honest answer out of a politician.
Also, the average voter lacks the ability to understand the complexities of a party's platform. I'd argue more politicians lack this ability, too, since a platform should reflect the party's stance on very complex, very different topics, from economics to foreign policy to education and job creation. Parties have whole teams dedicated to developing each aspect of the platform; John Q. Public is expected to understand it on his own. Of course people will resort to "dumbed down" delivery via 30 second commercials and political pundits.
Ultimately, this leads to a system where the ignorant *feel* ignorant, and either don't participate because they don't understand, or don't participate because they feel they have no stake in the game. Those who *do* vote have a vested interest in keeping their incumbent entrenched, and those who feel oppositely about the incumbent feel powerless to make any kind of political change.
I'd also suggest there's the idea that money equals success in America. The candidate that amasses the greatest fortune in campaign donations is seen as the most successful, despite any misgivings and indiscretions they may have had. Character is nearly meaningless in American politics anymore, simply because people don't trust politicians. Since character is meaningless, what other criteria do people use to base their decisions? Given the ongoing worship of the dollar, that those who are rich are upheld as success stories (no matter who they had to step on and over to get there), as role models, as aspirational and inspirational figures, it's no wonder most politicians are millionaires with little character.