Why has Fitch downgraded US credit rating from top level of AAA to AA+?

Fitch has downgraded US' credit rating from top level of AAA to AA+ amid concerns over the state of the country's finances and its debt burden. Fitch said it had noted a "steady deterioration" in governance over the last 20 years. US Treasury Secretary Janet Yellen said it was based on "outdated data" from the period 2018 to 2020.