The excretory system's job is to remove waste and excess water from the blood. Since Mark has diabetes and his body can't naturally do that, he has to undergo dialysis. Dialysis helps the kidneys function. And the kidneys are part of the excretory system.
I think it's because they didn't want it to be so easy for someone to step in and issue new decrees randomly. Everyone would need to agree, given that America runs on a democracy.
