'The System' is specifically designed to ensure this. All those built-in checks and balances designed to prevent one rogue element destroying the System through Extremism work precisely by giving those two Parties the bulk of the power, and to make it a waste of a vote to vote for anyone else. That's their purpose.
Most people think of this as a good thing, and if your System is working effectively then it is. You don't want wild swings in policy every few years, because even if a rogue actor with nefarious intentions doesn't get control those swings will shake your Society apart anyway. A System without those checks and balances wouldn't survive 100 yewars, never mind the nearly 250 the USA has lasted so far.
The problem arises when the System in place is not working. Those checks and balances that have kept it together for so long work to prevent any necessary changes as well as unwanted changes. There needs to be an effective way to make the changes needed without breaking the checks and balances. Unfortunately I don't think the USA has that, and it's problems have got worse over the last 50 or so years without anyone managing to make the necessary changes. Now those problems look like they are going to break the checks and balances, and with them the 250 year old System. What will replace it is anyone's guess at this point.
In short, the voters are only doing what they are supposed to do within the System they live. The System for keeping them in line and voting for the 2-Party System includes misinformation and propaganda in it's toolkit. People en masse have inertia, especially when they news and education systems are designed to feed that inertia. It's very difficult to break out of the miindset you've been conditioned into, and there is unlkely to be enough doing so to break the System. Those few with influence who have done so are never enough against the weight of Establishment-think arrayed against them.
Much as I despise Trump, in a lot of ways he's right. The system urgently needs drastic changes if it's to survive, It was rigged from the start, and that rigging is what will destroy it. I just don't think Trump intends to replace it with anything better,rather with a aworse, even less Free system than the one Americans have been conned into calling Freedom over the last 250 years.