The Skype software and infrastructure was changed in a way to abide the demands by the US intelligence agencies to allow access to all user communication.
source: https://www.forbes.com/sites/petercohan/2013/06/20/project-chess-how-u-s-snoops-on-your-skype/
archived source: https://wellthissucks.xyz/2013/06/20/skype-cooperates-with-the-us-government-to-allow-access-to-user-communications-archived-source/