Here's the answer: If a Q400 uses 1200kg/h than two of them use 2400, which is about the FF of a A320. With the same seat pitch you can load 180 pax on an Airbus/737.
Yeah, but with an A320 you'd have only one departure instead of two! There's an advantage to offering high frequency, which is possible with the Q400. Passengers have different schedules and needs. Also, if the 'bus is snagged, you'd be transferred to another flight much later, perhaps the next day even, instead of a short wait for the next Q400 flight. Imagine if the London Underground cut it's departures down to 25% of today's and quadroupled each train's capacity? Not necessarily a good thing...
Complex stuff...