There could be a reason for that. Both parties are great at turning off the electorate when they have the power.
Maybe that should tell them that declaring they have a mandate and trying to shove their agendas down our throats isn't the best idea.
Um, yeah, there's no reason why people should spend millions of dollars promoting an agenda and not doing anything about it.
Okay, reality check, for Pompous Mac.
Do you know why people get sick and tired of politicians once they are in office?
Because they realize nothing really changes.
All these wingnuts who vote for Republicans, and then find out abortion and gay marriage are still legal and nobody has rounded up that Mexican who is dating their daughter... that's why people get disillusioned.
Similarly, liberals (real liberals, not Islamophobic twats pretending to be liberals) vote to end the militarism and then they find out that they are still in Year 17 of a War on an Emotional State. The guy they elect is just as likely to use those pretty bombers as the guy they voted out.
Now, usually, this doesn't hurt the incumbant that badly, because people who voted for him aren't willing to admit they made a mistake. It hurts his party in midterms, and it hurts the poor schlub who picks up the check when it's over (Hillary, McCain, Gore).
The reality, though, is that 45% will vote for the Republican, no matter what. 48% will vote for the Democrat, no matter what. And 1% will vote for Third Party Assholes, because they are special and stuff.
So we are really fighting over 6% of the electorate.
The primary system does make both parties more radical because only the most committed (or ought to be committed) voters show up for them. Which is how we end up with an awful "Trump vs. Hillary" choice that no one really wanted, but yet 94% of the electorate voted for one of them at the end of the day.