Why does the US have such a low score for “Political participation” factor in the Democracy index?

According to Wikipedia's article on Democracy Index, the United States ranks 21st with the lowest indicators being "Functioning of government" and "Political participation".

I am wondering why such a relatively low score in the context of universal suffrage (virtually anyone who is more than 18 years old can vote and registration may be accomplished when obtaining a driver's license).

Question: Why does the US have such a low score for the "Political participation" factor in the Democracy index?