I'm very surprised nobody's ever run for President on the populist platform of "give you more choices" promising to call an Article V Convention, change the system, and break the duopoly. More than 60% of people weren't satisfied with their two choices in 2016, and thought they deserved better. It could be a winning issue.
The parties establishments don't want third parties for obvious reasons. They receive the lion's share of contributions from PACs and others because people like their money to go to a winner. If a candidate wants a serious chance of winning they have to please the party establishment in order to get that money and campaign infrastructure. Look at this election season when the debates were first starting, Johnson was only a few percent away from the minimum required to sit at the debates and there was a lot of pressure to let him in. The Commission on Presidential Debates is run by representative of the GOP and DNC so there was little chance of reform. Neither candidate said anything about it because, although it would have looked good to the people, they didn't want to anger their respective parties.