Final answer:
The decline in Medicare managed care enrollments after an increase was mainly due to the Balanced Budget Act of 1997, which resulted in reduced payments to Medicare managed care plans and insurers exiting the program.
Step-by-step explanation:
The legislation that was mainly responsible for the decline of Medicare enrollments in managed care after a rise in enrollments is the Balanced Budget Act of 1997. This act included provisions that resulted in reduced payments to Medicare managed care plans, which led to many insurers exiting the program or reducing their service areas. Consequently, the number of Medicare beneficiaries enrolled in managed care plans fell as beneficiaries returned to traditional Medicare fee-for-service plans or found fewer managed care options available.