I found this in the USITC website "In 2020, air freight experienced two different, but related impacts associated with the COVID-19 pandemic. The first, and most significant, impact was a sharp decrease in capacity to transport freight in the cargo holds of passenger aircraft (“belly cargo”) due to cancelled flights." https://www.usitc.gov/research_and_analysis/tradeshifts/2020/special_topic.html#:~:text=In%202020%2C%20air%20freight%20experienced,%E2%80%9D)%20due%20to%20cancelled%20flights.
What is the reason for this?