“WASHINGTON—Trying to restore lawmakers’ sagging image with voters, the Senate raced Thursday toward passing a bill to explicitly prohibit members of Congress, top aides and senior administration officials from insider trading and require them to publicly disclose financial transactions within 30 days instead of once a year.”
I think that’s because members of the House are frequently referred to as congressmen while members of the Senate are referred to as senators. Not sure when or why that started, maybe because Congressman X sounds better than Representative X.