It's always empty like that, even on holidays.
From what I've heard, Disney is contractually obliged to the Swan & Dolphin to run the dance club, as a service to its convention guests. My guess is that Disney sees the club as just an unfortunate and obligatory expense, like maintaining the roads to Bonnet Creek, and not as a real guest-oriented business that needs to make money.
To my knowledge, Boardwalk or Disney don't do ANYTHING that would help draw guests, like mention it on the WDW info channel or even put ads up in the Boardwalk elevators (classic Vegas tactic). It really seems like they don't want to spend any money promoting the place.