The Q400 is also able to carry a little more payload, 8747kg while ATR72 carries 7350kg
If you can get a full payload in it, maybe the dash can make more money.
SATA Air Açores witch now flies 4 ATP and a Dornier 228 (used to fly to Corvo, witch has a 800 meters runway) Is thinking about changing is fleet to a mix of different size dash8s. The ATR was also on the run, but for some reason the company is more likely to go with the bombardier aircraft.
The typical SATA flight is a 25-35 minute jump, these flights usually carry lots of cargo, and some of the islands have quite short runways 1300 to 1400 meters.
Maybe the dash is better fitted to this kind of operation, but as far as I know, no decision was made yet
RM