Both the Republican and Democrat parties are becoming more and more extreme with their views. I think at one time in the past, both parties were more in the middle and more "moderate" and willing to work together for the good of our country. That is NOT the case any longer. American politics has become more like a dick measuring contest. Both parties are full of nutty extremists. They lie to us, they make promises that they don't keep, they are bought and sold like prostitutes.
You are correct about neither party bringing what they promised. But both parties are not becoming more extreme, just the left has. I think the Republican party moved more to the left as well, just not nearly as much.
We don't vote because we have a favorite candidate, we vote to keep the worst candidate out. You can't do that by voting third party because nobody else would follow you.
I'm not voting for a Republican candidate this election, I'm voting to keep Hillary out.
I didn't vote for Romney, I voted to keep DumBama out.
I didn't vote for McCain, I voted to keep DumBama out.
So priority one is keeping the worst candidate out. That's why we will always be a two-party system in this country. The only way that will change is if both parties piss off their constituency at the same exact time. That's not likely to happen. The Republican party is now very divided, but the Democrats all march in lockstep and will never change.