"Possibly because the system for lodging it was opaque and challenging for accusers. Reade, a onetime Biden staffer, says she filed a complaint against him in 1993 when he was in his fourth term in the Senate representing Delaware. The process would have subjected her to a system that did little to protect Capitol Hill staffers from retribution and offered little recourse if they were not satisfied with the outcome. It would take a 1995 overhaul of congressional personnel laws to bring Congress in line with federal labor and anti-discrimination laws. Even almost 30 years later, the alleged complaint — the secretary of the Senate won’t even confirm or deny whether there is one — may never be released because of strict disclosure rules.... Four in 10 women who responded to a 2016 CQ Roll Call survey of congressional staff said they believed sexual harassment was a problem on Capitol Hill, while one in six said they personally had been victimized. 'Unfortunately, due to the system that Congress created to protect itself from being exposed, there has been no accountability,' [said Rep. Jackie Speier, D-Calif.]. Between 1997 and 2014, the U.S. Treasury paid $15.2 million in taxpayer dollars toward 235 awards and settlements for Capitol Hill workplace violations...."
From "The opaqueness of Congress’ workplace rules hangs over the Tara Reade allegations about Biden/Secretary of the Senate says law prohibits disclosure of any complaint" (Roll Call).