I believe there are three reasons our broken two party system stagnates and rots:

1. Our slavery founders intended for our system to be a plutocratic republic that would resist change, not a democracy

2. The purpose of every state is to maintain order which requires preserving itself

3. Fixing our system would require our politicians to vote against their own interests, to risk their own power and status... This goes against every ideal that capitalists believe in. Capitalists believe that as long as they follow the rules, they are entitled to everything they can take. The fact that the rules were written by the rich to favor the rich is of no concern to them.