Here’s the criteria that the score is build on.

From a quick glance, I’d say US is majorly lacking in:

• Equal opportunity to participate, vote: from felons bring denied voting rights to gerrymandering to Wisconsin votes being worth 4 times as much as California votes in presidential elections die to the EC, there’s a lot of fundamental issues here
• Independence of the judiciary: the supreme court is not independent, and presidential pardons are a mockery of any justice system