Of course the Q400's burns less, but when you start flying the Q400 on a longer stage length then the advantage in speed and time work in the favor of the E175's.
It works out in terms of speed, but that doesn't nearly offset the fuel burn. Take a 900 mile leg for instance. The Q does it in about 3 hours and burns about 6500lbs total, the E-Jet does it in 2:15 which is still a burn of about 8500 lbs. Sure passengers like the jet more and its faster but if the prop does it for 300 lbs less fuel and thus saves the company a grand a leg, guess which they are going with..