Compensation for substitute flights due to cancellation for "mechanical reasons"?

Wondering where I can find specific information on reasons why certain
airline flights have been cancelled, and whether airlines offer
compensations for inferior substitutions.

Last Saturday afternoon (10/16) My family and I were scheduled to fly
on USAirways 3210, nonstop from Savannah to Boston. However, the
flight was cancelled due to "mechanical reasons" which I highly
suspect was due to too few passengers on the plane. USAirways
re-routed a dozen or so of us flying to Boston first to Charlotte on a
mostly full Airbus, and then to Boston on a half-full 757.

I purchased tickets through Expedia, and if I had purchased tickets
for the one-stop flight in the first place, my total purchase price
would have at least two hundred dollars less. Would it be possible to
recoup some of our fares for being re-routed on the one-stop flight?