ELI5 Why is Medicare forbidden from covering anti-obesity drugs?

What was the rationale at the time the Medicare Modernization Act was being written?

edit: I understand the drugs available at the time had side effects, but why ban all future drugs might not have a severe side effects or might be more cost-effective why was the ban prospective?

edit #2: There are not yet any good answers below. There is some speculation about side effects, but Medicare has always covered plenty of drugs that have side effects.

edit #3: An answer! u/Minigoalqueen provided this article that says:

“There was skepticism of weight loss medications that existed at the time, and a sense that obesity was more of a behavioral problem than a medical condition,” says Juliette Cubanski, deputy director of the program on Medicare policy for KFF, formerly the Kaiser Family Foundation.