Your flight was, most likely, operated by Skywest or Expressjet. Not likely cancelled due to lack of passengers, but Skywest (which owns Expressjet) incompetence. There's a reason why UA was the most complained about airline in 2013, followed closely by Skywest and then Expressjet, which UA contracts for their regional jet flying. If at all possible avoid them as much as you can. $100 is not nearly enough for a flight cancelled due to poor crew management. Go to
www.united.com and in the upper right hand corner click on "customer care" (hah) and demand more compensation. Also, file a complaint on the D.O.T. website.