In 2004, The Walt Disney Company acquired the Muppets characters and franchise from The Jim Henson Company. This acquisition included characters like Kermit the Frog. As a result, the rights and control over the Muppets' appearances shifted to Disney.
Due to the change in ownership and licensing rights, the appearances of Kermit the Frog and other Muppet characters on "Sesame Street" became more limited. This was likely a result of negotiations and licensing agreements between Disney, Sesame Workshop, and other parties involved.