Because what he did four decades ago was unethical (and in some cases illegal) and reflects poorly on the institution he represents.
From what I can tell Moore is at the very minimum a creep, a predator at the worst.
Having said that we are giving Congress too much power to determine who will represent us DESPITE the will of the people. It's not that hard to see this committee being set up as a future political weapon by the establishment. In my opinion their jurisdiction should be restricted to the actions of SITTING politicians.
We are rapidly approaching a scenario where sexual misconduct or other bad behavior won't cost you your job but a simple accusation will.