Some routes may be too short to be financially rewarding using a jet (A319/320 or 737-300/700/800 or 717) due to low cruising FL; it may also only justify a smaller capacity aircraft (in the -72 to -78 seat range, such as ATR 72 or Dash8Q-400); a half-full A319 or 737-800 would not be profitable even if operated by FR or EZ. Last but not least, some of these regional routes link smaller regional airports where navaids, runways or facilities are limiting for heavy jets.
The recent high price of fuel seems to have effectively killed the 50-seater regional jet (EMB-135/145/CRJ-200) market and given a shot in the arm to modern turboprops like the Dash8Q and -500 series ATR. Bombardier themselves reckon that the Dash8Q-400's fuel burn is 28% lower than their own CRJ-700 on the same route.
Cheers
